The Sleeping Sharks of Isla Mujeres

The underwater caverns off Isla Mujeres attract reef sharks, a dangerous species. Once the sharks swim into the caves, though, they enter a state of relaxed nonaggression seen nowhere else. Naturalists have two explanations, both involving the composition of the water inside the caves, which contains more oxygen, more carbon dioxide, and less salt than usual.

According to the first theory, the decreased salinity causes the parasites that plague sharks to loosen their grip, allowing the remora fish (sharks’ personal vacuum cleaners) to eat the parasites more easily. Perhaps the sharks relax to make the cleaning easier, or maybe it’s the aftereffect of a good scrubbing. Another theory is that the caves’ combination of fresh- and saltwater produces a euphoria similar to the "nitrogen narcosis" scuba divers experience on deep dives.

Whatever the sharks experience while "sleeping" in the caves, they pay a heavy price for it. A swimming shark breathes automatically and without effort as water flows through its gills, but a stationary shark must laboriously pump water to continue breathing. If you dive in the Cave of the Sleeping Sharks, be cautious: many are reef sharks, the species responsible for the largest number of attacks on humans. Dive with a reliable guide and be on your best underwater behavior.