From June to September, large numbers of whale sharks—the world’s largest fish, typically measuring 6–7.5 meters (20–25 feet) and weighing more than 10 tons—congregate in shallow waters about 10 miles east of Holbox village. Despite their size, the sharks are completely harmless, eating plankton, krill, and other tiny organisms, much like baleen whales.

Snorkeling with whale sharks is a unique and (to some) nerve-wracking experience. The captain pulls the boat alongside a shark—at Isla Holbox they tend to feed on the surface—and two guests and a guide slip into the water with life jackets, masks, snorkels, and fins.

The water tends to be murky (it’s all the sea-life in the water that attracts the sharks in the first place), and the sharks are surprisingly fast. Still, you get a good view of these enormous, gentle animals, with their tiny eyes, bizarre shovel mouths, and dark spotted skin. It’s best to be on a small tour—since you go in two by two, you’ll get more time in the water.

Tours cost around US$80–120 a person, last 4–6 hours, and typically include snorkel gear, a life preserver, a box lunch, and nonalcoholic beverages. Some trips also include a stop on the way back for snorkeling or to visit the island’s inland lagoons and mangrove forest.