When he had been a boy, he had thought that stretch of the tunnel was glass because the view outside was beautiful. He had gaped through the train’s windows at the shivering fathoms of water outside, the floodlights standing like sentinels near the sulphur vents and on bare, rocky plains where nothing might have grown in a thousand lifetimes, the bits of light that living things made for themselves, darting quick and brilliant as finned fairies through the dark.

But he had grown up riding that moan of metal and magnetic rail through the dark, hearing it heave and labour as it hadn’t before, watching rust grow on the brackets that held each fragile glass section together. He knew now that the floodlights weren’t for the view, but because it was better for the creatures outside to attack them than the brief, tempting flashes of passing trains. He knew they weren’t riding along at the bottom of the ocean just because it was more beautiful than the surface no one had seen in over a century.

And he knew that stretch of the tunnel wasn’t glass so they could see outside, but because they had needed to scavenge every material they could in order to build and survive, and the weaker pieces had needed to go somewhere.