They were now in a steel-walled chamber about five meters in diameter. Its floor was concave, a flattened dome, so they had to mind their footing. Below the level of the port, at about knee height, it was ringed with an array of stainless-steel orifices. Other than that the walls were featureless to a height of about four meters above their heads, where a sturdy steel ring had been welded into place, projecting just a few centimeters inward from the wall. Directly above that was a flattish dome that completely sealed off the portion of the cylinder above it. "That thing moves. You’re looking at the underside of the piston," T.R. explained. "Down here, where we are, is where we explode shit." He bent down and tapped one of the orifices. "These are coaxial. They let in compressed methane and compressed air at the same time. They let it in fast to keep reload time to a minimum. When we get the amount we want, we plug our ears and spark it off with these." He pointed out a tiny detail Saskia hadn’t noticed before: little white ceramic knobs between the nozzles, with tiny metal parts projecting from the ends. "Plain old ordinary spark plugs from the car parts department at T.R. Mick’s. Volume discount."