Isla Mujeres

Every spring hundreds of whale sharks swim to Mexico's Yucatán Peninsula to feed off the nutrient-rich plankton in the area. The site is incomprehensible: dozens of gently curved, grey, polka-dotted whale sharks, most the size of a school bus, churning up the clear blue water. It's a playground of gentle giants—and it's safe to join. Whale sharks aren't a threat to humans—only to plankton—which makes swimming with them not only safe but a truly transcendental experience. And because the species is facing extinction, it sadly may be a fleeting one. The Blue Sphere Foundation, a nonprofit that advocates for oceanic health and endangered marine life, hosts dive expeditions off Isla Mujeres: You can see the whale sharks from the boat or snorkel among them.