The Cave Of Swallows at San Luis Potosi, Mexico. 333m deep, 135m wide. A sinkhole at the base goes down another 512m.
Vast, yes. Now consider this:
The cave floor is covered with a thick layer of debris and guano on which millipedes, insects, snakes, and scorpions live.
BBC’s Planet Earth explored the massive caves of the world back in 2006. Clips of the cave of Swallows which featured in the programme are not currently available online but if massive insect-infested guano piles are your thing, check out this excerpt from the programme featuring the base of Gomantong Caves in Borneo.