The walls of the cave were a slightly luminescent green, providing a helpful guide to what was an otherwise labyrinthine underground network. The paths weaved and wound in all directions, often looping back on themselves, such that several times Ryan was positive they were about to emerge back out to where they started.
The cave eventually opened on to a slightly larger room, but everyone stopped upon noticing that the floor had fallen away in front of them, creating a gap of several metres.
"That looks like a big drop," Yaz said, squinting into the unlit darkness below.
The Doctor looked around and picked up a nearby rock, dropping it into the hole and raising a finger so that everyone remained quiet.
"Six metres. Not that bad. Or at least, not deadly."
"You sure about that?" Graham asked, looking down to where the rock had landed, which was now giving out a gentle green glow around itself, illuminating dozens of furry legs that scampered from the light.
"More monkey spiders?" gasped Ryan. "What are they doing down here?"
"Great question. Also, why aren't they climbing up the walls?" asked the Doctor, no doubt rhetorically. "The walls aren't glowing like they do up here, which would suggest they're made from a different material. A frictionless one, at a guess. Which is good news for us."
"Unless we fall down there," Yaz corrected.
"Right, well, let's not do that then." The Doctor walked over to the side of the room and crouched down. "There's a small ledge sticking out from the wall, here. It looks like we could sidle across to the other side."
Her companions looked both sceptical and scared.
"Look, I'll go first and then I can offer a hand when I'm across."
Without stopping to hear the inevitable protestations, the Doctor eased herself onto the ledge facing the wall, and slowly but confidently sidled all the way to the other side.
"That wasn't too bad!" she shouted back. "I only thought I might die once. Who's next?"
The three looked at each other uncomfortably.
"You alright, son?" Graham asked Ryan. "I know this doesn't look easy for you."
"I'll be fine," Ryan replied unconvincingly.
"Well, how about I do it, and then if you can see how easy it is for an old codger like me, you'll be able to nail it, no worries."
Ryan nodded, and Yaz gently took his hand as Graham mounted the ledge.
"Cor, don't half make you realise how fat you've gotten," Graham sighed, pushing himself up against the wall.
"You can do it, Graham. I'm right here. There's no doubt in my mind that I can single-handedly support a middle-aged man if you slip."
"Real convincing, Doc."
Although he took his time, and risked looking below to the mass of writhing creatures, he too eventually made it across.
"Right…well…yeah," he managed, sitting on the floor next to the Doctor.
"I'll go," Yaz said to Ryan gently. "That way you've got all three of us ready to support you. Plus, you can't just turn around and run."
"You say that."
Yaz gave Ryan's hand one last squeeze before approaching the wall and gradually making her way across.
"Nice one, Yaz," Graham said when she dismounted on the other side. "Come on then, son. No worries, right? You can do this."
"I can do this," Ryan repeated, and approached the ledge. He looked down below him, the blackness somehow physically pulling him down, his head feeling like it might float away from the rest of his body. "Nope, no, actually I definitely can't."
Graham went as close to the edge as he could. "Hey, look, we're all here, right? I know you, Ryan. Better than anyone else here."
"Hey!" Yaz and the Doctor protested in chorus.
"I know that Ryan Sinclair is fearless. He doesn't let anything get in his way. Yeah, maybe his brain's wired differently from the rest of us and his coordination's a bit shot. But that's what makes him so brave. He wouldn't be the Ryan I know without it."
Ryan smiled and looked back at the ledge. "Guess I gotta live up to that now."
Slowly, Ryan placed one foot as securely as he could against the wall. And then, with slightly more confidence, his second.
"That's it!"
"You're doing great!"
"You got this!"
Ryan concentrated on every small movement he made with each of his feet, and found that by focusing on the minutiae, things became increasingly less overwhelming.
It felt like he'd be inching across the wall for hours, and Ryan was sure he must nearly be there, chancing a look to his side.
And then he fell.
His feet slipped away from the wall, his hands desperately trying to grasp onto anything, as the squealing from below became increasingly loud.
But then he wasn't falling. He looked up to see Graham grabbing his hand tightly, the Doctor and Yaz holding on to Graham.
"I've always got you, son," Graham smiled, and with the help of the others, pulled Ryan up to the rocky floor.
Everyone collapsed onto the ground, laughing away the fear.
"We did it!" Yaz smiled.
"Good job team!" beamed the Doctor.
Graham looked over to Ryan "How's that brain of yours now?"
"Total mess. But that's how I like it."