The Cave of the Sleeping Sharks

The Cave of the Sleeping Sharks (Cueva de los Tiburones Dormidos) is a series of caves and overhangs in the reef off Isla Mujeres, at roughly 18 to 20 m deep. In the late 1960s and the 1970s, sharks were found lying still on the cave floors, apparently asleep. The discovery caused a sensation because these were mainly Caribbean reef sharks, a species normally thought to need to keep swimming to breathe, lying motionless.

The story of the cave

Most accounts credit a local fisherman and lobster diver, Carlos García Castilla, known as "Válvula", with finding the sharks. A historical marker on Isla Mujeres dates the discovery to 1969, although some accounts place it earlier in the 1960s. García Castilla took the Mexican diver, photographer and filmmaker Ramón Bravo to see them. Bravo's photographs and films were so unexpected that, according to many retellings, some experts first suspected they were faked.

Bravo brought in the American shark researcher Eugenie Clark, who studied the sharks in the caves and wrote about them for National Geographic in the 1970s. Jacques Cousteau's team also came to film them, with Bravo as guide and cameraman. The cave made Isla Mujeres famous among divers.

Ramón Bravo spent much of his life diving and filming the Mexican Caribbean. He died in 1998, and his ashes were laid to rest in the Cave of the Sleeping Sharks.

Why the sharks rested here

The sharks in the cave were not truly asleep. Nurse sharks rest on the bottom all the time; what made the cave special was Caribbean reef sharks and other normally active sharks lying still, with their eyes following the divers. Clark and others suggested that fresh water seeping up through the limestone changed the chemistry of the water in the caves, with more oxygen and lower salinity, letting the sharks rest without swimming and perhaps helping to loosen parasites. Divers sometimes describe a shimmer in the water where fresh and salt water mix. The explanation was never fully settled, and you will hear different versions on the boat.

The dive today

Be realistic before you go. Many recent divers and guides say sharks are rarely seen resting in the caves now, and some suggest overfishing and visitor pressure drove them away. Today a nurse shark resting under a ledge is the most you can hope for, and even that is a bonus, not a promise. Shops that run the site sell it for the caves and the history, and some don't run it at all.

The dive itself is a deeper reef dive along a rocky reef with caves, overhangs and swim-throughs. Expect sponges, sea fans, groupers, snappers, angelfish and, in some years, lionfish, which are invasive here and which some local operators are allowed to spear. Because of the depth, your bottom time is shorter than on the shallow reefs, and nitrox helps if you're certified.

Who the Cave of the Sleeping Sharks suits

This is not a beginner's dive. Most operators recommend Advanced Open Water or equivalent, recent experience and good buoyancy control, both for the depth and because you are diving around overhangs, not in open water. You do not go deep inside: the caves are entered only as far as daylight reaches, and only where your guide leads. Cave diving beyond that needs full cave training and is a different activity altogether.

If you are newly certified and want to see the cave, consider doing it as the deep dive of an Advanced Open Water course with a shop that runs it regularly.

Shark etiquette

• If a shark is resting, keep your distance and stay below or beside it, never above it in its way out.
• No touching, chasing, flash at close range or blocking the cave entrance.
• Watch your fins near the floor: silt ruins visibility for everyone and disturbs anything resting there.
• Keep the visit short and let the next group see it too.

Historical dates and depths for the cave differ between sources. The ones above are the most common published figures.