Yucatán Cenotes

A near-perfectly circular, steep-walled open sinkhole south of Tulum — its signature feature is a hydrogen-sulfide cloud/halocline layer around 27-30m, created by decomposing organic matter, with submerged tree branches resting on top creating an "underwater river" illusion. Operators consistently state Advanced Open Water (not full Cave certification) as the minimum to view the cloud — the standard tourist dive has little overhead environment and no cave passages. Shore/platform entry, no liveaboard.

A cavern system (not full cave — divers stay within sight of natural light throughout) split into the Barbie Line (a ~500m daylight circuit past stalactites, stalagmites, and columns) and the Bat Cave (surfacing into an air dome with hundreds of resident bats). Open Water certification suffices for the standard guided tour; no separate cavern specialty required. Shore/platform entry, day trip from Tulum or Playa del Carmen, no liveaboard.