Cenote Calavera — the Skull Cenote, also known as the Temple of Doom — sits just outside Tulum Pueblo on the road to Cobá. Three openings in the limestone form what looks like the eye sockets and mouth of a skull when seen from above. The largest opening is wide enough to leap from, and a wooden ladder lets you climb back out.
The cavern below holds a chamber large enough to swim around in with a torch, and there is a halocline — the meeting layer between fresh and salt water — at about ten metres depth, visible as a shimmering blur to snorkellers. The site is small and rustic, with little infrastructure beyond a hut where you pay entrance. Bring your own snorkel gear.