The sound boomed out again for five or six seconds, and this time, they all saw it. Right where Paul had touched earlier, a section of glass slid down, as if it were simply melting open, creating a rectangular hole the size of a typical door. Inside, filling the entire cylinder, something huge and dark zoomed past like a train, going at an incredible speed. Tick couldn’t see any details, scarcely believing that whatever it was could move at such a velocity.

The train thing was gone as soon as it had come, and the glass melted upward, closing the door and reforming until not a single blemish or mark revealed it had ever been there.

“Whoa,” Paul said.

“This must be a tunnel for some kind of bullet train,” Sofia said. She gingerly reached out to where the doorway had appeared, then tapped the glass with her fingertip and pulled away. “It’s not any hotter than the rest of the tube.”

“We’re obviously supposed to go inside,” Tick said.

“And get smashed by that thing?” Paul said. “Wasn’t much room for a nice stroll in there if that train comes flying by again.”

Sofia turned toward the two of them so they stood in a small circle, facing each other. “Tick’s right. It can’t be a coincidence that we showed up here next to this big tunnel, right where a door opens up. We have to go inside.”

Paul shook his head. “Well, I’m not too keen on the idea of getting run over by a monster train. That door seems to open only every half-hour or so and it only stayed open a few seconds. Jumping in there sounds like the worst idea I’ve ever heard.”

“There has to be a path and a railing, right?” Tick said. “Even if it’s small. Any subway in the world has a walkway, doesn’t it? For people to make repairs and stuff?”

Paul shrugged. “Maybe, but it sure seemed to me like that thing was right next to the glass.”

“Yeah, it was,” Sofia agreed. “But what else are we going to do? Sit out here in the sun and bake to death? There’s no sign of anything for miles and miles except that stupid chair—I guess we could try sitting on it again, but—”

“We have to go in there,” Tick interrupted, nodding toward the tube, knowing he was right.