"Anyways, we settled on hydrogen. Some of it’s gonna leak out and burn, but it’s a desert, so who cares? It’s easy to make more, on-site, from natural gas. So, at the same time the methane-air mix is filling the chamber below, we fill this volume with a certain amount of H 2 . When the combustion chamber"—he pointed straight down—"goes boom, this piston we’re standing on gets forced upward, compressing the hydrogen. Which only has one way out." He drew their attention to the top of the cylinder, ten meters above their heads, where it tapered inward like an upside-down funnel to an orifice in the middle about the size of a manhole. If the entire cylinder was a bottle, that was its mouth. "Let’s see where it goes!" he suggested, touching off another round of extension cord wrangling, hatch closing, and ladder climbing that took them up above the top of the massive steel cylinder to Minus One.