Harry was starting to measure his life's progress by the easy skill-ups he got from finding books in the library, so he was a little disappointed that in the next two weeks he only managed to get through another twenty shelves. Between increased homework requirements and Oliver Wood's frankly insane quidditch training schedule, it seemed they hardly had any time at all to just be in the library, browsing books.
They did remember Hermione's birthday and managed to fit in a bit of a celebration for it on the 19th, to her great pleasure at being remembered.
At least he was getting increasingly-positive feedback from his teachers as his relevant skills began to reach the point where he should be by the end of his second year. He was worried he was going to crash when he finally ran out of skill books and had to go back to improving the normal way. At least he was clearly still getting XP in the skills from class instruction, and had ranked up his flying skills from all of Wood's practice.
Of course, he'd also missed a few hours in the library for their new elective project: trying to figure out where the Chamber of Secrets was. The original Marauders had never found it, but they had to use their own measuring skills to try to account for dead space in the walls (as they'd determined from a couple more conversations with Sirius and Remus). The new Marauders had already filled in a few more secret passages that their forebears had written off as the castle's tendency to move, or in places they couldn't accurately pace off the depth of a classroom.
While lots of parts of Hogwarts needed overly-thick walls to bear the mountain of stone above, it was easy enough to start to notice when dead space was suspiciously thick. This was especially true if you had a holographic map that could precisely render the known space in the castle. Every time they finally figured out the entrance to a passage, it would replace the fog of war on his map. And, as a bonus, it got filled in on the parchment copy of the Marauder's Map as well. Somehow.
Trips to the dungeon were the riskiest part, since it was Slytherin territory, but even the snakes weren't much willing to harass a crew of a half-dozen Gryffindors led by Fred and George looking like they were on a mission. In that way, they'd finally mapped off the crazy block of rooms and shaft up to the third floor that Dumbledore had used to create the previous year's obstacle course protecting the Stone. It used wizarding space to expand it, since it was actually a fairly small block of rooms when viewed from the outside. They even found a secret door that led into the room that once held the troll, explaining how it had been put there in the first place. It probably also explained why it had made sense to the staff that there had been a "troll in the dungeon."
They got into the back door so easily that they were just hoping Dumbledore had let the spells sealing it lapse. Otherwise, Quirrell could have easily skipped the first few obstacles. The fact that the rooms were largely empty seemed to support the idea that it had been cleaned out over the summer. Fred, George, and Lee enjoyed getting to at least see the space they'd heard the stories about, and Ron was interested to see the rooms he'd missed after his chess-based concussion.
There were still a few spaces they were working on trying to get into (mostly because they were behind staff offices or other house common rooms where they couldn't really loiter while poking at walls and trying passwords). But the most suspicious were a large blank space in the middle of the seventh floor and what appeared to be another shaft like the drop into the obstacle course, but which took up contiguous space starting in the second floor and going down as far as they could get.
"It has to be the shaft," George argued, where they'd convened in a little-known passage near the Gryffindor common room.
Fred nodded, "It's either a trap door in the third floor girls' loo, or you walk into the wall on the second."
"It's weird that it's in the loo," Lee opined. "You'd think they would have found that when they put in plumbing. How long could the castle have even had plumbing? Not since the founders, right?"
Hermione brightened, "Maybe they did find it. If I remember correctly, Hogwarts: a History says that the plumbing project was finished in the 1940s. Maybe whoever framed Hagrid was working on the toilets and found the passage?"
"So we're saying this was an evil plumber?" Harry checked.
Ron shook his head, "I bet they made students do it. Probably pretended it was a big fun project for extra credit in transfiguration."
"That would get Prefect Percy to do grouting," Fred grinned.
"Or our Hermione," George added, patting her on her voluminous curls.
"It would be an interesting project," she objected, swiping his hand off her head. "Anyway, I am still worried that there's clearly a large space just missing so near our dormitory, but I concur that it's most likely the shaft. Especially since it's adjacent to Myrtle's toilet."
"That's where I saw the dot disappear!" Harry finally realized at her prompting.
"Not it," Fred and George said simultaneously.
"Not it what?" Harry asked.
"They don't want to interrogate Myrtle," Lee explained. "Big babies."
"Easy for you to say," George insisted.
"She likes you," Fred added.
Lee shook his head, "She'd like you if you hadn't spent all of second-year tricking her and Peeves into fighting."
"It was epic," Fred grinned.
"Flooding in all the corridors. Everyone covered with gunk," George nodded.
"Anyway," Lee rolled his eyes. "Why don't I go talk to her? Been meaning to see if she can leave the castle to go to Hogsmeade."
"You're going to invite a ghost to Hogsmeade?" Ron boggled.
Lee shrugged, "She'd be cute if she wasn't so down on herself all the time. And, you know, was alive. I figure it will be good practice for asking living girls out."
"We'll wait outside the loo," Fred said.
George smirked, "Maybe we'll pick up some pointers for our own dating."
They couldn't really hear much, loitering outside of the second-floor girls' bathroom. But Lee exited a few minutes later and gave them a thumbs-up. They retreated a little ways down the corridor and he explained, "Nobody's ever asked Myrtle how she died. I think the basilisk got her."
"You're burying the lede, man!" George objected.
"Yeah," Fred followed-up, "Is she going to Hogsmeade with you?"
Lee shook his head, "She was very touched, but she can't really leave the castle. And…" he glanced significantly at Harry, "she says she has a crush on someone else."
"I wonder what kind of perk I get from dating a ghost," Harry mused, using that thought to cover for how annoyed he was that even ghosts were hero-worshiping him.
"Anyway," Lee bulled on, "she says she was in that loo and she saw a big pair of yellow eyes, and that killed her. And she thinks they were coming out from the sinks. I looked over at them and one of the taps has a little snake etched into it. She says that sink has never worked."
"She's right," Hermione nodded. "It's like it's not even hooked up… everyone knows to avoid that one. Well, we mostly avoid Myrtle's loo completely, but sometimes you're in a hurry and have to risk it."
"And the sink's right in front of where the shaft down to the Chamber of Secrets should be?" Harry checked.
"Exactly," Lee said. "It's some kind of secret door. I figured we could all go in and work out how to open it."
"We'd better go ahead," Hermione decided. "It's almost curfew and with classes tomorrow, who knows when we'll get another chance."
They nodded and trooped back down the hallway to the bathroom, not noticing the office door they'd been talking in front of crack open and a perfectly-tousled head peer out after them. Gilderoy Lockhart had picked an excellent night to catch up on his fan mail. Later, Harry would be kicking himself that he'd gotten into the habit of turning off his map unless he was actively looking for something. To be fair, it did obscure the upper quarter of his vision.
The second-floor girls' bathroom was similar to the one in which they'd fought the troll on the previous Halloween, except suffering much-worse maintenance (especially after the third-floor one had to be completely repaired due to said troll fight). The sinks were chipped, the paint was peeling, and the place was lit by sputtering magical candles (as opposed to the bright magical torches used throughout most of the rest of the school). One of the stalls had the door broken off the hinges, and the glasses-wearing ghost known as Moaning Myrtle was hovering over the toilet. "You brought him!" she said, almost cheerfully, to Lee.
"Of course. We need to see if the Boy-Who-Lived can figure out how to avenge your death," Lee said, gallantly.
"Well, if he wanted to do that…" she mused, drifting out of the stall and into Harry's personal space. That close, he realized that she was dressed as a student, and had probably died not much older than he was. "He'd go track down Olive Hornsby and punish her. She was the reason I was in here to die."
"I guess you can't go haunt her yourself?" Harry checked.
Myrtle let out a low whine and agreed, "It was great for a couple of years. But then she graduated, and somehow she had me stuck at Hogwarts. Death isn't fair!"
The rest of them had quietly started inspecting the broken sink and found the tap marked with the snake. While the twins poked and prodded and Hermione was clearly doing mental geometry trying to figure out where the latches might be, Ron suggested, "Heir of Slytherin would have also been a parselmouth, right?"
Hermione agreed, "Hogwarts: a History mentioned that Salazar Slytherin could speak to snakes. I guess it's hereditary?"
"I think so," Ron said. "And you'd have to, if you wanted to control a giant snake. So maybe the password to open the door is also in parseltongue?"
"I'll try!" Harry said, eager to get an excuse to put some personal space back in between him and the simpering (and also slightly whimpering) ghost. He located the tap, spotted the snake, and ordered, "Open up."
"English," Ron told him.
"Harder than it looks," Harry grimaced. He thought about the time he'd talked to the snake at the zoo, and tried to imagine the etching of the snake was a real snake, ordering, "Open."
"That's done it," Ron agreed, as everyone stepped back from the magical white light emanating from the tap and the grinding of the sink, well, sinking into the floor. It revealed a huge pipe in the wall exactly where they expected the shaft to be. "That's big enough for a person to slide down," Ron figured.
"Or a very big snake," Hermione's eyes widened.
"Should we… slide down it?" Harry checked.
"As fun as that would be…" Fred lit his wand and peered down into a grimy tunnel that descended well out of view of the light.
George nodded, "It might be a chore getting back up again."
"I've got my broom," Harry shrugged, happy for his inventory system yet again.
"But we'd notice someone else with a broom walking around school," Hermione realized. "The snake probably comes up, if you call it."
"It can't be that easy?" Harry figured. "We just call it up, let out the rooster, and we're done?"
"Of course it will be that easy," George disagreed. "All good plans are simple."
"But just in case, we'll be waiting outside," Fred nodded.
Lee rolled his eyes again at his best friends, "And we'll be ready to put a locking charm on the door. Just run out if it's a problem."
"See," Fred clapped Lee on the back, "this plan even has layers!"
"Just, you know, don't get petrified before you run out," George smirked.
"I better be getting most of the XP for this," Harry stuck his tongue out at the boys. "But, yeah, it doesn't make sense for us all to be in here to get petrified at once. Myrtle, you should stay out of sight too."
"Maybe I want it to kill me again," she objected, but not strenuously since it was a pretty good day for her, emotionally. "Fine," she ultimately agreed, and jumped into her toilet, disappearing into the pipe with a splash of water.
"Good luck, Harry," Ron said as the others cleared out into the hallway.
"Don't get petrified," Hermione ordered as she closed the door behind her.
"No pressure," Harry sighed to himself. Making sure he had his inventory open to his menagerie to be able to quickly grab the rooster, he leaned down over the giant pipe. Once again imagining he was speaking to a snake, he yelled down the pipe, "Hello! Come to me, King of Serpents!" It was interesting that the word "basilisk" translated that way when he used parseltongue.
"Speaker! I am coming," that same terrifying voice echoed up the pipe. "Which foul cretin unworthy of magic are we killing today? Let me rip! Let me tear!"
Harry gulped, stepped back to give himself as much clearance in the room as possible, and wondered if maybe this was the dumbest idea he'd ever had. He didn't even have a sword. It felt like you should have a sword, if you were fighting a giant monster.
He had a slightly-traumatized rooster. As the murderous rantings got closer and closer, he hoped that would be enough.