They pulled a few more lice out of the ridges of the whale's throat and one particularly fat one from its blowhole, and then bade it goodbye. The giant animal rumbled in gratitude, and Luca could have sworn he saw it smiling as they waved to it. His mother might pass out when she learned they'd gone so far from the shallows, but that was definitely something he'd do again if he got the chance.

On their way back, he took the lead, homing in on exactly where they were going even though there were no landmarks visible. Some of what he'd learned in school had suggested that this came from an ability to sense the Earth's magnetic field, but that was one of many, many things he hadn't yet had time to learn more about.

"What did you think about that, huh?" Alberto asked on the way. "Both of you?"

"That was amazing," said Giulia. "The lice were kind of gross, but being that close to a whale... I never thought I'd get to do something like that! What did you think, Luca?"

"I can't believe I didn't think of this before," said Luca, who was still focused on what he was thinking about. The source of the hot water. It was so obvious...

"I can understand whale language," Alberto bragged. "Well, some of it. My Dad knew more. He used to tell me what they were saying, so when it's easy stuff like hello or thank you I can get it. I always kinda hoped he'd teach me some more, but... yeah." He paused a moment, and decided he didn't want to talk about this anyway. "Now dolphins, on the other hand, dolphins can talk normally when they want to, but they usually don't. I think it's because if they're just chirping at each other they can talk about us when we're right there."

"What do whales and dolphins talk about?" Giulia asked, fascinated.

"Same things anybody talks about," Alberto replied with a shrug. "You know, what your family's up to and what the currents are like and where the sardines are schooling. And humpback whales will try to get you to listen to their poetry. They write these poems all about their trips from the south pole to the north. They take hours to recite and they're so boring."

"Maybe I can learn some whale," said Giulia. She would be the first human to ever do so... if that counted anymore, which maybe it didn't. "There is so much cool stuff down here. Why did you guys even want to come visit us up on land anyway?"

"Stars!" Luca replied immediately.

"Pasta!" Alberto said at the same moment.

"Weather."

"Gravity."

"Books."

"Vespas! But seriously," Alberto added, "we're only showing you the fun stuff. Most of the time it's really boring down here. That's why my Dad and I lived on the Island, where there was interesting stuff."

Giulia laughed. "The grass is always greener, huh?" She couldn't imagine anything in this underwater world being boring. It was going to be weeks before anything above the surface interested her, if it ever did again.

While the farming community of sea monsters relied on the sunlit shallows for their crops, the blacksmith lived further from shore and correspondingly deeper. His forge was in a sort of miniature trench that cut into the continental shelf from the south, perhaps a hundred and fifty metres deep. Halfway down the east side of this was a space where big, irregular chunks of volcanic stone had collapsed together, leaving an open space below. Light was visible through the gaps between the boulders, and further up the hill what looked like black smoke was rising, accompanied by a sulphurous smell.

The water got hotter as the three kids approached, and by the time they peered through one of the openings into the cavern it was as warm as a bath. Inside, the open space was lit by dozens of luminescent jellyfish clustered by the ceiling, gently pulsing as they glowed pink. The black smoke could be seen gushing out of the floor near the back of the cave, then flowing up into the chimney that took it out into the open sea. A variety of stone and metal tools were hanging on the walls, and a big, squared-off block of basalt near the chimney formed the anvil.

The blacksmith himself was not using any of that right now. Instead, he was looking into the mouth of a two hundred kilogram seal that was lying on its belly on the anvil stone. The blacksmith glanced up and saw that he had visitors, and held up a hand to tell them to stay where they were. Luca, Giulia, and Alberto therefore waited quietly.

"Okay. Ready?" the blacksmith asked.

The seal nodded nervously and shut its eyes.

"Don't try to bite me this time." The smith reached into the animal's mouth, fastened a metal clamp around something, and yanked. The seal yelped and blew bubbles out its nose, and a cracked tooth came out, along with a wisp of blood that quickly dispersed in the water.

"There we go!" said the blacksmith. "That wasn't so bad, was it? Go up and take a deep breath. You're fine."

The seal dashed out one of the openings in the ceiling and made straight for the surface, without anything that might have counted as a thank you.

The blacksmith tossed the tooth he'd just extracted into a stone bowl, and came to the entrance to see his visitors. He was muscular and potbellied, and intimidatingly large – if he'd been standing up on land, he would probably have come close to matching Massimo's towering six foot seven. Rather than being brightly coloured like many of the other sea monsters were, he was slate grey, with dark eyes and thick fins that made him look like a chubby shark. His smile was friendly, but decidedly on the toothy side, even by sea monster standards.

"Hello, Luca," he said. "I haven't seen you in ages! Your folks said you were at school up on land?"

"Yes, Signor Donzella," Luca replied politely. "I'm just visiting for the holidays. How is Signora Donzella?"

The blacksmith winced. "She and Junior are still staying with her brother... she says down here is no place to raise a child. At least she left me Ottavia," he added, with a glance at a little mottled octopus that was suckered on to the stones of the chimney, napping in the warm like a cat in a sunbeam. "Alberto, you've been keeping busy, too?"

"Always," Alberto agreed.

"I haven't seen your Dad around in a while... what's he up to these days?"

"I don't know, and I don't care," was the cheerful reply.

"Oh. I... see." Donzella look at Giulia. "And who's this? We don't get a lot of new people around here."

"Giulia Marcovaldo," she introduced herself.

"Silvestro Donzella," the blacksmith replied. "What brings you kids down here?"

"We want to know about the hot water." Luca slipped through the gap in the boulders and into the cavern itself. The water in there was so warm that it was uncomfortable – a human in a room at a similar temperature would definitely have been sweating. "Where does it come from?"

"Well... it comes from below," said Donzella. "I don't know why it's warm, if that's what you're asking, but it's like the vents in your kitchens, only much hotter..."

"Because it's closer to the source!" Giulia finished, wriggling through to join Luca. She had a moment's pause at finding out just how hot the water was in here. It was like walking into the Patisserie kitchen in the middle of July, with all the ovens open and the heat nearly unbearable. The unpleasant thought occurred to her that this must be what a lobster felt like as it was tossed into the pot.

"The source?" asked Donzella.

"The volcano," Giulia clarified.

"The Mediterranean is at the junction of the Eurasian, African, and Anatolian tectonic plates," Luca explained. "See, the crust of the Earth is made of a lot of big slabs of rock that rub against each other, and where that happens, they heat up and made volcanoes and earthquakes. Can we take a look outside?"

Donzella clearly hadn't understood a word Luca just said. "Um, of course," he said, "but be careful of the smoke. It's far too hot to touch, and the sulphur will make you ill."

"Thank you!" Luca darted outside again.

His friends followed him as he made his way up the steep side of the trench, pulling small corals and handfuls of organic slime out of the gaps between the stones until he found something that didn't feel like basalt.

"Guys! Look! Look!" he called.

Alberto and Giulia came closer to peek over his shoulders. He'd found a deep gap between two of the slabs of stone. The surfaces were eroded, softening their corners, but there had once been corners there.

"Feel that," he ordered, pointing into the gap.

Alberto reached in rather gingerly – that crack looked like a good place for a hungry eel to hide, and a big enough one wouldn't be against snacking on sea monster fingers. What he found, however, was a different sort of stone, made of pebbles embedded in something solid. Giulia took her turn next, and was startled to recognize the texture.

"It's concrete," she said. "Luca, remember that broken barrier at the railway station in Genova? It looks smooth on the outside, but where it had cracked you could see all the little stones they used as filler."

"That's exactly what I was thinking," Luca agreed. He turned to Alberto and pointed into the gap again. "This is artificial. Somebody built this hillside to take the black smoke away, so Signor Donzella could work in the forge without choking on it or getting scalded by it."

"Built it?" asked Donzella himself, who had followed them out to make sure they didn't get too close to the chimney. "I don't know if I believe that. Look at the size of these stones. You couldn't move those."

"Humans could," said Luca firmly. "They've moved all kinds of huge stones to build things, like the Colosseo in Roma."

"And there's concrete in that, too!" Giulia added excitedly.

"You think humans built this?" asked Donzella, more confused than ever.

"No," said Luca, "but if they could build stuff like that on land where there's gravity, we could definitely build it in the water. I bet the water for the kitchens comes from here, too! The conduits will take it to our houses, and on the way it cools down enough that it doesn't heat them up like it does the forge!"

"You'd have to clean it, too, or it'd make the food taste nasty," Alberto observed – all three kids could taste the sulphur in the backs of their mouths, even though the chimney opening was still metres above them. "How do humans clean their water?" The water that came from the taps and fountains in Portorosso was definitely not the same as the muddy stuff that flowed out of the culvert, or the salty seawater in the harbour.

"I don't know, but I bet Mamma does," said Giulia. "Her mother used to work for the utilities board in Genova. We can ask her when we go back for lunch."

The three did indeed have plenty to tell Helena when they returned to the Pescheria for sandwiches and fruit around one in the afternoon.

"Sorry we're late!" Giulia said, hurrying through the doorway. Although the weather was sunnier, she was still shivering in just her bathing suit. "We talked to a whale! Alberto understands some of their language."

"I do!" Alberto said with a nod.

"And we visited the sea monster blacksmith, and this afternoon we're gonna decorate our Christmas sponge!" It was still tucked in among the rocks near the Island.

"You've been busy," Helena observed, passing out sandwiches with porchetta and tomatoes. "I wouldn't have thought they'd have a blacksmith underwater."

"He uses all this black smoke from an underwater vent," Luca said. "Hey, did Arturo make it home okay? His sister was looking for him."

Helena sighed. "I asked him if his family knew where he was and he said they did, so I had him help me with some laundry before he left. He seemed awfully excited about it."

"Everything's exciting when you're doing it in a new place," said Giulia with a grin. "Luca, remember how excited you were to help with the dishes, the first night Mamma asked you?"

"You were just as excited when Mom had you help do it last night," Luca reminded her with a smile.

"Oh, really? Am I allowed to hope you'll bring that newfound enthusiasm home with you?" Helena asked her daughter. Giulia giggled.

"Signora Marcovaldo," said Luca, "do you remember how they cleaned the water, when your Mom worked for the utilities in Genova?"

As far as Helena could tell this question had come out of nowhere. "The water?" she asked.

"We think the hot water for cooking comes from the same place as the blacksmith's smoke," Luca explained, "but it's all sulphury."

"A long time ago, the sea monsters built conduits from a volcano to bring it to their houses," Giulia added, "like how the Romans built the acquedotti."

"I see." Helena thought about it. "If I remember correctly, the drinking water in Genova comes from springs in the mountains. I"m not sure it even needs filtering, but I think that's mostly done using layers of sand and charcoal."

"Do you think maybe the Romans taught us about concrete?" Luca asked Giulia through a mouthful. "You said they used it in the Colosseo, and you mentioned pictures of us in the mosaics."

"Maybe they did," she said, "or maybe you taught it to us."

That hadn't even occurred to Luca. "You think so?"

"If it sets underwater, we definitely came up with it first," was Alberto's contribution. "Humans dry things to set them."

"Like pottery," Helena agreed, "or paint."

"We have paint," Alberto said, "but it's a lot thicker than your runny stuff."

"We definitely need to go to the bookshop," said Giulia. "We need to find stuff on filtering water, and on concrete, and maybe we can find something on Oannes... but it's probably better to go to the Biblioteca in San Giuseppe." This was a larger town on the other side of the hilly peninsula, where people from Portorosso often went when they needed something they couldn't get locally.

"I don't know if I want you going all the way to San Giuseppe," said Helena.

"I've done it before," Giulia reminded her. "I can ride my bike there and back."

"We'll go with her," Luca agreed.

"Yes, but that was... you know, before this." Helena lowered her head. "I just... I'm sorry, Giulia, I can't stop thinking about you ending up in a zoo or a laboratory," she admitted. "If you want to go to San Giuseppe, let me know, and I'll go with you on the bus, okay?"

"Okay," sighed Giulia.

"We won't do that this afternoon, anyway," Alberto put in, "because I still have to show you my shipwreck." He was getting impatient about it. Yes, they'd showed Giulia some cool stuff, but the shipwreck had been one of the original reasons they'd wanted the camera, which had led in its sideways fashion to this situation. "You want to talk about moving big rocks, there were some really big rocks being moved on that ship."

"And we have to decorate the sponge, remember," Luca said.

"And then it's dinner here tonight?" Helena asked. "For all of you?"

Luca squirmed a little in his chair. "Not for me. Uncle Ugo is visiting, so Mom wants me to talk to him a little, but I'll come here after."

Alberto and Giulia exchanged a worried glance. It really didn't seem fair to just leave their friend to face this man he found so intimidating, even if his family were going to be there, too. "Maybe I could stay, too," Alberto suggested. Giulia had already promised to be home for dinner, but they'd let Alberto eat with the Paguros if he wanted.

"You don't have to do that, Alberto," said Luca firmly. "Mom wants me to talk to him, and if you're there I'll just end up talking to you instead. I'll be fine." He offered them a nervous smile.

Despite this promise, Luca didn't have much to say as the kids returned to the water to go see Alberto's shipwreck. He was too busy wondering what Uncle Ugo was going to want to talk to him about, and whether he would be encouraged to eat any more worms or bits of whale carcass or whatever else it was people ate in the Deep.

Giulia made up for it – she was annoyed, and she was going to talk about it. "Mamma's not usually a worrier, that's what Papà does," she fumed. "She knows I've ridden my bike from here to San Giuseppe before... and she's let me and Luca go all over Genova on our own! Remember when we had to go to three different bookshops looking for that one book Signor Bruzzone recommended?" She had to pause and get rid of her lungful of air as she went under, so that she could speak in the water. "And now she's worrying like I'm six years old! Does she think I'm gonna forget how to do human things just because I'm learning to do sea monster things?"

"She's afraid people are gonna treat you the way my Mom is afraid humans will treat me," said Luca.

Giulia scowled – but she did know what he meant. There was an old movie that the little theatre in town had used to play, called Creature from the Black Lagoon. They didn't show it any more now that the sea monsters liked to come to town, since they thought it might offend them – the titular creature did look very much like a sea monster, just without the tail. That had just been a silly monster movie, but in the cinema in San Giuseppe Giulia had once seen the sequel, in which the creature was captured and taken to an aquarium to be displayed to everybody and studied by scientists. Even at the time she'd felt kind of sorry for it. She'd almost forgotten about that movie, but now it suddenly seemed very scary indeed...

But that was dumb, she realized a moment later. If such a thing happened to one of them, they could just climb out of the water and what would people do then? They couldn't treat what was apparently a human being that way, could they? Santa Mozzarella, they couldn't even treat a sea monster that way if they knew it could talk. If something talked, that made it a person.

"Shipwreck!" Alberto declared, determined to get off this unpleasant subject. "This way!" He forged ahead in the water to lead the way.

The continental shelf off the Ligurian coast was mostly shallow and gently sloping, in a marked contrast with the hilly landscape surrounding Portorosso. There were, however, a few steeper places, like the gorge where Signor Donzella had his forge, or where the seafloor rose up to become the Island. Alberto led his friends past that – their Christmas sponge was still waiting for them there, although an inquisitive cuttlefish was taking a look at it – and down a slope to the bottom of an undersea hill. At first, there didn't seem very much to see there except a row of boulders, but those boulders were in n awfully straight line.

Once the kids got close, they were able to see what had gotten Alberto's attention: the boulders were not randomly-shaped natural stones, they were big barrel-like cylinders that represented five broken sections of what had once been a single immense column. Alberto settled on the seafloor next to one.

"The boards of the deck all got eaten by worms," he said, "but I'm pretty sure it must've collapsed when the ship sank anyway, because if you dig a little you'll find that the stones are lying right on top of a whole bunch of broken pottery." He pointed south – even down here, where the water was beginning to get dark, all three knew which direction that was. "I think that was the front of the ship. The really neat stuff, though, is here at the back." He moved to behind the last boulder, and began digging in the silt. "Here! Here's something!"

Alberto held up the object he'd uncovered. It was a Roman brooch missing its pin, shaped in a gentle curve to hold the thick fabric of a toga or cloak. Having been buried in mud, the silver had turned black, but had not fallen apart the way it would have in sea water.

"Can I see?" Luca took the object from Alberto and turned it over, then held it up with a smile. "It's a dolphin! Here's the eye, and the dorsal fin!"

"Anything you find, bury it again," Alberto warned them. "The man from Savona said it'll keep them from deteriorating."

"Got it," said Luca. He handed the brooch to Giulia and moved on down the line of boulders to look for more things.

Giulia turned the metal object over in her hands, taking in the details Luca had mentioned, then carefully put it back in the hole Alberto had dug. Had that been part of the cargo, she wondered, or had a sailor been wearing it when the ship sank? Were they going to find ancient bones down here? Probably not... not after two millennia. At least, she hoped not.

The sections of column were not so well-preserved as the objects that had been buried in the mud. Surfaces which once must have been smooth had been roughened by many centuries in the salt water, and barnacles and algae had colonized the sides. Between two sections Giulia found a piece of pottery that she at first thought was one of the broken jars Alberto had mentioned, but when she dug it up, she found it was a clay oil lamp. It was also the home of a crab, which reached out a claw to snap at her for disturbing it.

"Sorry, I'll leave you alone," she said.

She moved on to the next cylinder, and then the next down the line. Odd little corals and sponges were growing in between them, and little shrimps were crawling around. When she reached the last section, she wondered if she would find the fancy top the Romans had used to put on columns. There were three kinds, and she remembered that the most elaborate was called Corinthian but could never recall the names of the other two. Beyond the final section, the ground dipped down a little further. There was a roughly square chunk of stone there with stuff growing all over it, which might indeed have been a capital.

Sitting on top of it, facing away from her, was the genie.

Giulia hadn't expected to find anybody else down here, and definitely not the genie, which she had assumed had gone back to... wherever genies came from. But no, it was sitting there with its legs crossed and its chin in its hands, staring thoughtfully into the dark – until it suddenly turned to look at her with a puzzled expression.

"Hello again," said Giulia.

"Are you looking for me?" the genie asked.

"No, we're just here to see the shipwreck," Giulia replied. "We didn't mean to bother you."

The boys, hearing the other voice, came to see who Giulia was talking to. Both were as surprised as she to find the genie here. Alberto leaned on the last stone cylinder with a puzzled frown, but Luca immediately came closer.

"It's you!" he said. "Can I ask you something?"

But the genie was still talking to Giulia. "What do you want to see in this old wreck?" it asked.

"Alberto wanted to show it to us," she replied. "There's a professor from Napoli coming to see it."

"I was hoping," Luca said, "that you could tell me more about Oannes."

The genie continued to ignore him. "I would have thought you'd be at the sunken libraries of Tyre by now," it said to Giulia. "Or learning sorcery from Scylla and alchemy from Charybdis! Are you hoping to find some of my siblings?"

"No, we just wanted to see this place," said Giulia. She was getting a little concerned now. It hadn't occurred to her to wonder what the genie would do after it vanished, and now it seemed like the genie didn't know, either. "Are you okay? Do you have anywhere to go?" If the shipwreck were from Roman times, that meant the genie had spent nearly two thousand years stuck in that bottle ont he sea floor. Maybe it had no idea where to go. Maybe it had found the world changed beyond all recognition and didn't know what to do.

"We could find you somewhere to stay," Luca offered. "There won't be space at my house because Alberto and my Uncle are both staying there... maybe on the Island?" He looked at Alberto.

"Sure. As long as you don't mind it's kind of cold this time of year," said Alberto. "And don't mess with my stuff too much. I spent a long time collecting it and fixing it up."

"You could come back to Luca's house and help us decorate our Christmas tree," Giulia suggested, thinking that might help the genie feel more at home.

"What's that?" the genie wanted to know.

"On land it's a pine tree with ornaments on it," Luca explained. "For underwater we've got a big sponge and some pretty shells."

The genie frowned, puzzled. "Why?"

Giulia couldn't remember anyone ever asking such a thing. Christmas trees weren't something there really was a reason for. "It's just what people do for Christmas," she said.

The genie stood up, and floated in the water just above the stone it had been seated on, the way it had hovered in the air in Giulia's room. "Let me see if I understand this," it said. "You wished to be a member of the race that spawned the greatest scholars and magicians the world has ever known... and now you're poking around in sunken ships and hanging shells on a sponge? That's all?"

"That's not all," said Alberto, in the voice he used when he was about to say something deliberately obnoxious. "Earlier today we picked nits off a whale."

The genie shook its head. "But when you made the wish," it insisted, still looking at Giulia. "Surely that's not all you had in mind, was it?"

Giulia didn't answer. The genie seemed weirdly upset by the whole idea, and the only answer she had to its question was probably going to make it worse. She hadn't had anything particularly in mind at all. She'd just wanted to hang out with her best friends in their element. Maybe that was a dumb reason for wishing to be transformed into something else, but she'd been having way too much fun to regret it.

"Our roe was hatching," Luca answered instead. "She wanted to see."

The genie threw its hands in the air. "By Marduk's teeth!" it exclaimed. "I figured the Children of Oannes would kill you for your presumption, but you almost deserve it! The sheer insolence, treating their magic like a plaything..."

"You thought we were going to kill her?" Luca interrupted, horrified. "She's our friend!"

"You didn't say anything about that!" Giulia said. It obviously wasn't true, but it seemed like a thing the genie ought to have mentioned.

"You didn't ask. You only asked if I were going to do you any harm or trick you," the genie pointed out.

Luca was determined to get his question in one way or the other. "Hey," he said, "I've been wondering. You keep calling us Children of Oannes. Who is that?"

The genie stared at him. "Who is that? Who is Oannes? Oannes! The founder of your dynasties! The king who made the first treaty with the humans! How can you not know Oannes? Have you learned no history?"

"I go to school with Giulia in Genova," said Luca. "They don't teach sea monster history there."

"Then why do you bother to go?"

"Because sea monsters don't have schools," Luca said.

"Where do you live that they don't have a school?" The genie grabbed Luca's arm. "Show me!"

"Uh..." Luca swallowed and pointed up the hill. "My house is up that way..."

In a flash, they were suddenly back in the pastures near shore. It was a disorienting experience, which everybody's ears popping and stomachs heaving from the sudden change in water pressure. The genie let go of Luca and spun in a circle, taking in the little farmsteads and the fallow fields. Signora Zigrino cried out in surprise at seeing them appear from nowhere, and dropped her urchin basket.

"This is the middle of nowhere," the genie said. "Where are your cities? Where are your temples? Your libraries?"

"I... don't think we have any of those," said Luca.

The genie stared at them a moment longer, and then simply vanished, leaving a scatter of sand drifting to the sea floor.

Signora Zigrino came swimming over to check on them. "What was that all about?" she asked. "Are you three all right?"

"We're fine, we're fine," said Alberto.

"We're okay," Luca agreed. "Giulia, you're okay, right?"

"Yeah," she said, "I am," but it wasn't quite true. The genie had said no catches, but it hadn't seemed very bothered by the idea that it was sending her to her death. What had really shaken her, however, was hearing that she had insulted the sea monsters and deserved to be hurt. Nobody had ever told her something like that. Even Ercole, who had said a lot of awful things to her over the past few years, had never said she deserved to die. She felt ill thinking about it.

"You're looking a little green around the gills, Pesciolina," Signora Zigrino said. She looked around, then plucked a leaf off a particular piece of seaweed. "Suck on this. It'll settle your stomach. Don't swallow it."

Giulia put the leaf in her mouth and found it had a strong sharp taste, like a sour apple. It did seem to do something, though – her shivering subsided fairly quickly, and the queasy feeling in her stomach went away.

"Thank you," she said, with the leaf still in her mouth.

"Don't mention it. I know you're new around here," Signora Zigrino said. "I'm the local erborista. Any time you're not feeling well, come and see me, all right?"

Giulia nodded. The genie didn't know what it was talking about, clearly. Sea monsters were friendly creatures – the first thing Signora Zigrino had done was offer help! Then again, Signora Zigrino just thought she was new around here. She didn't know Giulia used to be human. They hadn't told Signor Donzella that, either, because they didn't want to have to explain over and over. But... maybe it was better not to.

"Come on." Luca took her hand. "Let's get the sponge and go back to my place."

Giulia spat the leaf out before they set off. She wasn't feeling physically ill anymore, but she was still... distressed, definitely. You deserve it wasn't something anybody could quickly get over hearing.

"Hey, don't listen to that dumb genie," said Alberto. "I don't know what he was on about, but nobody's mad at you."

"Yeah, we're glad you're a sea monster now," Luca agreed.

"So am I," said Giulia firmly.