The Riviera Maya is a great area for outings. A unique feature of the area are the "cenotes" (seh-no-tays), usually translated as the unalluring word "sinkholes". Cenotes are pools of water which form in the soft limestone that's common in this region. Sometimes they're underground; sometimes they form a swimming hole, as above.
The "sinkhole" term refers to the formation: groundwater seeps through the limestone, creating a pool in an underground cavern; eventually the roof of the cavern gets so thin it collapses, and an open pool -- like this swimming hole-- is formed.
photo © Teresa Plowright