The lift slowed as it approached 215 meters below ground level. Their view was improving as, it seemed, some of the routed systems got routed elsewhere. It was now possible to see the six steel barrels of the gun arrayed around them. The lift eased to a stop. Through a window in its door they could see into a well-lit chamber beyond. It wasn’t a large space but it was definitely external to the main shaft. "See, down here at the bottom we did a little more blasting and hollowed out some extra volume," T.R. said. He opened the door and led them into the side shaft. It was not much larger than the bedroom Saskia had slept in last night on the train. The walls were bare limestone, still bearing the marks of the pneumatic tools that had carved them out. The floor was a grate through which they could look straight down into another story below. No wonder she’d been warned not to wear heels. Two men and a woman stood against the back wall wearing air tanks on their backs connected to respirator masks dangling free on their chests. "Safety first!" T.R. said, waving at them. "This area has plumbing for four different gases, three of which can kill you. One, natural gas—basically methane, which we get for free from a well right here on the ranch, about ten miles away. That is the fuel we burn to power the gun. Two, air, which we use to burn the methane and which, incidentally, keeps us alive. Three, hydrogen, which is the light gas we compress to drive the projectiles up the barrels. The air in the peashooter. We obtain it by cracking natural gas in a plant upstairs. Either hydrogen or methane would cause an explosion if it were to mix with air, as the result of a leak or malfunction, and be ignited by a spark. So, four: as a backup system we have stored down here a large quantity of compressed argon, which is an inert gas that does not support combustion. Or life. Now, to be clear, the methane and hydrogen lines have been totally disconnected. We’re not stupid. There’s no way such a leak could happen right now. If one were to occur, it would be detected by these here doohickeys." He indicated a white box mounted to the stone ceiling that looked like a smoke detector as designed by the Pentagon. Saskia now noticed more of them all over the place, merrily blinking. "If that were to happen, an alarm would sound and argon gas would flood this whole volume and drive out the air and render the atmosphere non-combustible."