The Cave of the Sleeping Sharks (La Cueva de los Tiburones Dormidos), Isla Mujeres
Updated: Aug 11
About a 30-minute boat ride from Isla Mujeres is a phenomenon that captured the world's attention. Sharks, thought to be sleeping, were found in caves by a local lobster fisherman, "La Valcula". This famous dive has bought the curious to the island for decades to see this strange behaviour Jacques Cousteau documented with Ramon Bravo in 1970.
The History
1969 was the year that Isla Mujeres was put on the map, just a short time before the massive development of Cancun commenced. A local fisherman, Carlos Garcia Castilla (known as the "Válvula" because of his exceptional lung capacity), was hunting for lobsters in an area known as Los Cuevones (The Big Caves) when he realised that a particular cave had sharks constantly entering but not coming out. This first contact with the sharks was a great fright to him because he came out of the water terrified.
Driven by curiosity, he returned a few days later, freedived into the cave at 20mt /65ft to see what was happening and discovered a most unusual scientific phenomenon. One that would draw international attention to Isla Mujeres. In the cave, he saw what appeared to be sharks resting on the sand. Little by little, he approached the sharks until he realised they were actually "sleeping". He returned to Isla with great doubts because, as far as he knew, sharks did not sleep, or at least not that anyone knew of.
Before this discovery, it was assumed that sharks didn't sleep because they needed to swim to obtain oxygen. Also, it was thought that sharks had to be constantly in motion because they lacked a swim bladder. Only Nurse Sharks/ Carpet Sharks (Ginglymostoma cirratum), which have a slow metabolism, had previously been scientifically studied while they rested. Until the discovery of Carlos Garcia, sharks with faster metabolisms had not been seen at rest.
When Carlos Garcia returned to Isla Mujeres, he sought out Ramon Bravo, a respected island resident.
Ramon Bravo was a well-known Mexican diver who had worked with Jacques Cousteau. He was also an ecologist, cinematographer, author, Olympic swimmer, and, most importantly, a dominant figure in the worldwide preservation and study of sharks.
At first, Bravo did not believe Carlos and thought it must be harmless nurse sharks crawling on the sand. Still, Carlos Garcia kept insisting until finally, Bravo went to verify the discovery.
Apparently, the first days were frustrating and unsuccessful due to bad weather. They had to wait until it passed to enter the water and did not find anything in the cave for many dives. Doubt set in regarding the validity of what "La Valvula" saw. Fortunately, on a subsequent day, in poor conditions, they saw an enormous bull shark apparently totally asleep in the cave.
Inside entry to the vast cavern, spread out over several galleries, at least 20 sharks of many species slept peacefully. Bravo could not believe what he was seeing, and he went on to verify the discovery in the following days taking many pictures and videos of tiger, mako, whitetip, leopard or bull sharks resting peacefully on the white sands of the cave floor. What surprised everyone was that every shark was in front of the current that penetrated the cave.
Ramon Bravo shared the phenomenon with the Smithsonian Institute, but initially, his photos were thought to be fakes, and he was denied the discovery.
Fortunately, it attracted the attention of international scientists Dr Shelton Applegate, a master in shark paleontology, and Dr Eugina Clark (aka The Shark Lady), a member of the Scripps Institution of Oceanography of La Joya, California and a world authority on sharks. The pair immediately organised to travel to Isla Mujeres to study this extremely unusual behaviour.
A month later, Ramon Bravo and Valvula entered the caves of the sleeping sharks again but this time with Dr Clark. On this occasion, Valvula carried an underwater explosive rifle, Ramon his camera and film equipment and Dr Clark a couple of assistants to take note of the position and behaviour of the sharks.
Dr Clark stated that
"Due to their static position on the sand against the current, the sharks remained immobile and practically "drugged", because when the water ran, they took the oxygen necessary for their respiratory process through their gills, without the need to move, the salty water mixed with fresh water from the Yucatan Peninsula producing an electromagnetic field in the water, in addition to helping to deworm the dormant sharks." (1)
This research was featured in National Geographic and earned Dr Clark a gold medal from the Society of Women Geographers.
Jacques Cousteau, with his son Philippe and his divers, arrived on their research boat Calypso in 1970 and started diving immediately with Bravo and Valvula to investigate The Sleeping Shark Cave. Ramon Bravo was a guide and cameraman for the sleeping sharks and the reef surrounding Isla Mujeres.
What resulted was the documentary The Sleeping Sharks of Yucatan, in which Bravo appears in the credits.
Jacques Cousteau promised to give Carlos Garcia (Valvula) credit, but shortly after his arrival, he wanted to take credit for the discovery. This proved problematic, but fortunately, Ramon Bravo had already published images, always giving full credit to Valvula.
These images of the Caves of the Sleeping Sharks and famous lobster runs travelled the world and surprised the scientific community. Soon National Geographic wanted to be involved.
On more than one occasion, Ramon Bravo suffered for his passion and dedication to filming sharks. He was once attacked by a bull shark that bit him on the left forearm at the elbow. He required urgent surgery, where he received 54 stitches and was in intensive care for several weeks.
The Sharks
Although the cave is called The Cave of the Sleeping Sharks (or Sleeping Shark Cave as some call it), it is unclear whether the sharks actually sleep. Ramon once stated that the shark's eyes followed him intently as he moved around the cave.
The water inside the caves contains more oxygen, more carbon dioxide, and less salt than usual, leading to two theories as to why the sharks enter a state of relaxed nonaggression that has not been seen elsewhere.
The first theory is related to decreased salinity.
Remora fish live in a unique symbiotic relationship with the sharks, feeding on the scraps of prey dropped by the shark, but more importantly, they feed on the parasites on the sharks' skin and mouths. It is thought that the decreased salinity causes the parasites to loosen their grip, allowing the remora to eat the parasites that plague the sharks more efficiently. Sharks may relax to facilitate the cleaning, or maybe it is just the aftereffect of a darn good scrubbing.
The second theory is that the combination of freshwater and saltwater in the caves, due to underground freshwater springs, may produce a euphoria similar to the Nitrogen Narcosis that scuba divers experience in deep dives.
The Legends
On 21 February 1998, Ramon Bravo passed away, having contributed so much to the world. He suffered a heart attack caused by an electrical shock in his home in Isla Mujeres.
His ashes were laid to rest at the cave entrance of the sleeping sharks by his wife, Maria Vallejo, and priest, Eduardo Perez. Many were in attendance, including the then-president of Mexico, Ernesto Zedillo, his dear friend Jean-Michel Cousteau and many authorities.
His ashes rest under a bronze plaque at the entrance and read:
Ramón Bravo, protector of the sea and the ocean, sleeps forever next to his sharks in this cave. Isla Mujeres 02–28–98.
Carlos Garcia Castilla passed away in 2017. With his death, we said goodbye to another legend who had made the most incredible discoveries.
A historical marker was erected for him and can be found in downtown Isla Mujeres here.
The Dive
Sadly, it is not common to see sharks in the cave now. Overfishing has taken its toll, and the shark population is struggling to return. Fortunately, there has been a noticeable increase in shark sightings lately, as eco-tourism is superseding much of the fishing industry around the island, and much is being done for their protection. Still, it is a long and slow process.
The cave is about a 30-minute boat ride from the island, and the Cave of the Sleeping Shark lies at a depth of 20mt/ 65 ft.
It is a popular site for divers, but many are disappointed when they do not see any sharks. The dive is still incredible but not recommended for inexperienced divers.
Depth
20 meters / 65 feet
Boat Ride 30 min
Required Experience Level
Advanced Open Water Certified Diver / Experienced Diver
Average Underwater Visibility
20 meters / 65 feet
Average Water Temp
26 Cº / 79 F°
Normal Dive Time
35-40 minutes (safety stop required)
To view the fish surveyed here with reef.org, click HERE.
(1) Wikipedia contributors. (2022, June 30). Ramón Bravo. Wikipedia. https://en.wikipedia.org/wiki/Ram%C3%B3n_Bravo
Maestro Fernando May. (2019, November 16). El Mexico Que Se FueLA CUEVA DE LOS TIBURONES DORMIDOS Y LA LEYENDA DE ’ ’ EL VALVULA ’ ’ [Facebook article]. Facebook. https://www.facebook.com/permalink.php?story_fbid=pfbid0FwTu9f9f26znxEbxLbbTdLkzWG6fCCWDaa8mYiKd4ukakgAhNbTS5acdiTCHw76Bl&id=511450782396272
Cave Of The Sleeping Sharks, Cancun. (2022). TripHobo. https://www.triphobo.com/places/cancun-mexico/cave-of-the-sleeping-sharks
History of Isla Mujeres. (2015). Copyright 1997–2017, Isla Mujeres Net. https://isla-mujeres.net/history.html
Ramón Bravo. (2022). Wikiwand. https://www.wikiwand.com/en/Ram%C3%B3n_Bravo
Recent History. (2021, May 15). Isla Contoy Tours. https://contoyadventures.com/isla-mujeres/recent-history/
Remembering Ramon Bravo: Environmentalist, Oceanographer, and Olympic Athlete. (2015). Isla Mujeres Magazine. http://islamujeresmagazine.blogspot.com/p/by-ronda-winn-roberts-ramon-bravo.html
S. (2022). Sleeping Sharks Cave / Tiburones Dormidos. Sea Gypsy. https://sea--gypsy-net.translate.goog/diving-spots-catalog/item/sleeping-shark-cave.html?_x_tr_sl=it&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=sc
The Shark and The Remora Fish – A Unique Relationship! (2019, April 15). North Shore Shark Adventures. https://sharktourshawaii.com/blog/shark-remora-fish-unique-relationship/
Sleeping sharks in the Mexican Caribbean. (2015). Amura World. https://amuraworld.com/en/topics/conservancy-report/articles/2511-tiburones-dormidos-del-caribe-mexicano
The Sleeping Sharks of Isla Mujeres - Isla Mujeres. (2022). Fodors Travel. https://www.fodors.com/world/mexico-and-central-america/mexico/isla-mujeres/experiences/the-sleeping-sharks-of-isla-mujeres-2816156
S. (2021, November 8). Ramon Bravo: Early Life, Sleeping Sharks & Net Worth. Celeb Doko. https://celebdoko.com/ramon-bravo/