Surrounded by two oceans and the Caribbean Sea, Mexico awaits your deep dive, where you experience crystal-clear waters, amazing undersea life and incredible rock formations. You'll wish you never had to surface. Top Mexico diving: just off the world's second largest coral reef (only Australia's Great Barrier is bigger) Cozumel Island, in Quintana Roo, is a bit of pure heaven, with gentle, sparkling currents, millions of colorful fish and a brilliant sun that keeps everything warm and rugged. Or try the Yucatan's inland grottoes and underground rivers, called cenotes—the perfect break from the peninsula's intense sun and heat. On the west coast, a dive around San Pedro Nolasco Island, in Sonora, gets you up close and personal with sea lions and seals.

In crystalline Caribbean waters, you gently float, dive and even lay hands on magnificent, tranquil whale sharks. The majesty of the animals and the vastness of the sea offer real understanding of man’s place on the planet. Top snorkeling with whale sharks in Mexico: whale sharks stay close to mysterious, unspoiled Holbox Island in Quintana Roo all summer long. When you come up for air, you find trees and lagoons filled with flamingoes, pelicans and other tropical birds.

Going underground in Mexico is a true thrill, in dark, intriguing caves and cenotes that have only just begun to be explored. Expert guides make sure there's a light at the end of the tunnel. Top Mexico spelunking: Many consider San Luis Potosi's Sotano de las Golondrinas the most beautiful open-shaft cavern in the world, and you can hardly disagree as you contemplate its jarring, at-surface entrance and a first, freefall drop of more than 1200 ft. The lush jungle setting and hundreds of squawking parrots—your companions all the way down—are like something out of science fiction.

The world’s grandest, most enigmatic mammals—including endangered fin-back, gray and blue whales—love Mexico just as much as you, and return each year to the Sea of Cortez for mating and feasting on the bay’s abundant plankton. Top Mexico whale watching: the birthing habitat at Magdalena Bay in Baja California Sur or expeditions setting out from Los Cabos (also in Baja Sur) or San Carlos, Sonora.