A burn pit is exactly what it sounds like: A hole is dug in the ground and filled with trash, including medical waste, vehicles and plastics. Then the whole mess is doused with jet fuel and set on fire. Operated in large part by KBR, the huge contracting firm that was enriched by lucrative wartime contracts, burn pits were a ubiquitous feature of the post-Sept. 11 wars — a primitive disposal method for the tremendous loads of garbage generated by occupying forces.