The Yucatan and surrounding region into Guatemala is built up by limestone that dissolves with acidic runoff from rotting vegetation. This causes the region to be riddled with underground caverns like Swiss cheese.
In the Yucatan, these caverns are called "cenotes" and can be have a very thin roof between them and the ground surface. If the roof collapses, you get a sinkhole.
While there, we spent a morning swimming in Cenote Dzitnup that was illuminated by a round hole in its dome from a partial collapse.
