The flashover simulator is like a metal trailer parked out on the wide concrete apron behind the burn building. It's divided into two sections, one mounted slightly higher than the other, so the interior has a ground level area and a platform level about three feet higher. The idea is that you build a fire in the raised section and can experience it from the slightly cooler (non-lethal) pocket of air below the platform level. Add sufficient fuel for this fire, in the form of wooden pallets and compressed wood panels, and it will build to the point of flashover.

Flashover, we had been lectured, is the point when the heat from a fire within a room becomes so intense that everything inside suddenly bursts into flame. What is really happening is that the heat is causing all the combustibles to emit flammable vapors, and the heat buildup causes these to catch fire. Basically, the air itself burns.

If you are caught inside a room when it flashes, your equipment will keep you alive for about four seconds. Since you're crawling on your hands and knees to stay below the intense heat, that typically means that you only have a chance at survival if you're within five feet of a door or other means of exit. The flashover simulator allows trainees to experience an actual flashover, and get a better understanding of the stages that lead up to it and how (if you are equipped with a hose) it can be forestalled long enough to get out.

We sat through a last-minute session to go over what the flashover would be like and how to avoid getting hurt in it. It was a sobering lecture. "Do not touch anyone else in the flashover simulator," the instructor told us. Our gear creates small airspaces around the body that insulate us from the heat. "If you slap someone on the shoulder to get their attention, you can leave a hand-shaped burn right there."

We would use special helmets and SCBA face pieces to avoid damaging our personal gear. "If your facepiece starts to melt or deform, let one of the instructors know," we were told. The helmets were wrapped in once-shiny heat-resistant material, like relics from a 50's sci-fi movie that had been thrown into a campfire.

Everyone stood near the flashover trailer getting their gear ready while the instructors prepared things. This was probably the first situation in which I could be burned if my gear wasn't totally together, and I obsessively checked and rechecked the places where gaps could form. We gamely joked about making sure everyone sealed the Velcro fly of their bunker pants. In Emergency Medicine, a burn to the genitalia is tallied as 1% of the total body surface area, but it is considered a critical burn. "Protect the package -- don't get the critical 1% burn" we laughed, and tried not to be nervous.

We went into the trailer in a group of five. I think there were three instructors in there, and they had pulled in a charged hose line as well. The atmosphere was a little smoky, but we had not yet attached the regulators of our SCBA. The main instructor told us what to look for -- early clean burning, then a descending level of thick smoke, and then... fire, coming right over our heads as we crouched in the lower level of the simulator. At that point, we would be given the chance to hit the fire with short bursts of water from the hose to see how it could temporarily stop the flashover.

I was first one the hoseline, on my knees next to the instructor at the base of the platform level. We clicked in our regulators and began breathing off our SCBA. He applied a long-handled blowtorch to get the fire going, and I watched as it consumed a box of wooden planks. The fire began to slide up the wall behind it, catching a wooden sheet and growing. I could already feel the heat through my face shield.

Gathering strength, the fire reached the roof level and began rolling out across the ceiling. There was still very little smoke. The flames were like a luminous fluid spreading out over the ceiling; it was like watching from above as a bottle of fire was poured into a box. As the wooden planks in the walls and ceiling caught fire, heavy smoke began to descend. We were almost completely blind. I could hear the instructor yell at me to watch the space over our heads.

A thin snake of fire unfurled and curled out into the air above me. Another shot by to my right, coiling and shimmering like an aura in the smoke. The instructor put his hand into the air and waved, and fire plumed in the wake of his hand, like bioluminescence in a boat's wake on a nighttime ocean. Then more flames shot across the space above us, crossing and intertwining like something ancient and alive, and all at once there was fire everywhere. The fire's breath was an enormous, dry roar. I saw the silhouette of the instructor's helmet against a rolling sheet of flame that consumed the space just over our heads, the air itself shuddering and giving way to fire.

"Now!" the instructor yelled, "hit it quickly, left right, and center!" I opened up the hoseline for three brief bursts, aimed at the ceiling level, and the fire lifted off us and evaporated. I felt the wave of thick, hot steam, and what little visibility I had immediately went to zero. But it had worked.

"Rotate!" the instructor yelled, and I crawled back around to the rear of the trailer and allowed another trainee to take my place at the hose. Already the fire was building again, regathering and preparing to flash. Again the thin tongues of fire unfurling through the dark air, and then the sliding tide of flame spreading out overhead. I watched, utterly captivated, this living thing I had never really known before.