The cave systems were air-filled passages and chambers, ventilated by the same processes that circulate air through any dry cave system.

Into these dry caves, during the last Ice Age, fell animals.

The karst landscape of the Yucatán is dotted with natural sinkholes — called cenotes — where the ceiling of an underground chamber has collapsed, opening a vertical shaft from the surface into the darkness below.

These shafts acted as natural traps.

Animals that walked too close to the edge fell in.

The drop was often fatal, and even animals that survived the fall found themselves in a chamber from which there was no exit.