This is the difference between archaeology and tomb raiding. If it were tomb raiders, they might have to risk their lives against the poisonous gas, but for Leonard, he could slowly wait for the smell inside to dissipate.
"I think someone must have died here," Fairmont said from inside the tent, "The smell of a decaying corpse differs from other rotting things."
"I think we could have guessed that without even trying. Just think about how we already dug up six skulls outside, and the ones that were shattered may probably piece together 2-3 bodies. Based on this kind of death, I suspect that most of the Japanese devils probably died here."
Compared to their previous few explorations, Leonard and the others clearly didn't need to retrieve the bodies of those who were already dead, so their work was considerably easier this time.
