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With more than 2,000 square miles of tropical forest, marshlands, mangroves, and coral reefs, Sian Ka'an Biosphere Reserve is among the largest protected areas in Mexico. The Cancún-based nonprofit conservation group Amigos de Sian Ka'an (011-52-9-880-60240) can provide information and leads one-day boat tours of the area for $55 per person. Admission to the reserve is free.
Getting There: Round-trip flights from New York to Cancún start around $550 per person on American Airlines (800-433-7300, www.aa.com); from Los Angeles, $670 per person. Tulum, on Sian Ka'an's northern border, is 80 miles south of Cancún; from there, follow the rutted dirt road 40 miles to Punta Allen. Alamo Rent-a-Car at the Cancún airport (800-462-5266; www.goalamo.com) can set you up with a four-wheel-drive vehicle starting at about $420 per week.
Where to Stay: The Cuzan Guest House in Punta Allen (011-52-9-834-0358; members.aol.com/fishcuzan) caters to fishermen and offers guide services for $350 per day. Their nine thatch-roofed cabanas with ocean views go for $40 to $80 per night, double occupancy. Camping in Sian Ka'an is allowed only in designated areas; for details, contact the reserve office at 011-52-9-849-7554.