Natural charms and ancient allure set Riviera Maya a lush world apart

Eight of us - a half-dozen visitors, our guide, and the boat's captain - are headed for the far shore, which from a distance presents itself as a low wall of mangrove. Our destination is the Cayo Venado, a 6 3/4-mile waterway that snakes from the brackish coastal lagoons to a series of large freshwater inland lagoons - the same route used by Mayan traders 2,000 years ago to move goods from the jungle to the sea.

We reach the other side, and Cosme, our captain, a short, stout Mayan man wearing wrap-around sunglasses, guides the narrow boat into a break in the mangrove, the mouth of the Cayo Venado. We're suddenly surrounded by vegetation, and the sky seems to shrink a bit. Tall, flowering bromiliads wave high above us. The farther upstream we travel, the narrower the passage becomes. Mangroves give way to expanses of savannah grass as the water becomes fresher. Ben, our guide, points out a pair of nesting osprey, and a large dark turtle tumbles off the bank into the stream. Cosme swings us around a bend and the stream narrows again, so much so that the lower branches of the scrubby savannah trees reach from bank to bank. Gliding beneath them is like momentarily being swallowed by this vast, elemental landscape.

Which is why we've come. We're on a daylong tour of the 1.3-million-acre Sian Ka'an Biosphere Reserve, the largest protected natural area in the Mexican Caribbean. Established by the Mexican government in 1986, the area is home to hundreds of bird and mammal species, from roseate spoonbills to jaguars, crocodiles, spiny lobsters, land crabs, and a teeming population of lizards. There are 23 known archeological sites, some more than 2,000 years old; Sian Ka'an was named a UNESCO World Heritage Site in 1987.