Whale sharks, the largest fish on earth, can grow as long as 45-feet. Plop into the water beside one of these slow-moving, polka-dotted, school bus-sized behemoths. No matter how many times you tell yourself they only eat plankton, it’s still a thrill. Especially when they open their five-foot wide maw to suction in a cloud of krill. The Earth’s biggest concentration of the rare fish occurs every May to September off Mexico’s Isla Mujeres, a 20-minute ferry ride from Cancun. Each morning 10-passenger boats from Ceviche Tours carry snorkelers to sea in search of whale sharks ($125). The fishermen who run the outfit have arrived at their own code of conduct to limit impact—only two swimmers are allowed in the water at a time and each boat spends a maximum of 30 minutes with any individual shark—so you’ll take shifts all morning slipping overboard to kick alongside the silent, gliding beasts. Bring a camera, take a picture, and upload it to the ECOCEAN Whale Shark Library to help scientists track just how far these behemoths travel.