TIKAL NATIONAL PARK, GUATEMALA: The overused term "lost world" finally feels appropriate when you first glimpse the Mayan ruins of Tikal, once a thriving metropolis of 100,000 people that peaked around a.d. 700. Temple IV, Great Plaza, and South Acropolis, the major ruins in this 143-square-mile park in northern Guatemala, poke out of a mist-shrouded canopy, while toucans flutter, monkeys chatter, and coatimundis cross your path. Visit when the park opens at dawntrails of vapor rise from the ruins like departing spirits. You'll find the rustic, backpacker-friendly Jaguar Inn (doubles, $48; 011-502-926-0002, www.lacasadedondavid.com/index.html) near the entrance to the park.
FORT JEFFERSON, DRY TORTUGAS NATIONAL PARK, FLORIDA: The seven islands that make up the Dry Tortugas National Park70 miles west of Key Westwere discovered by Europeans in 1513, when Ponce de Léon arrived and named them after the sea turtles that fed his sailors. The islands are still known for their marine life, but the ruins of Fort Jefferson, on 16-acre Garden Cay, are the main attraction. Construction on the red-brick fort began in 1846 but was never completed. Reach Garden Cay by seaplane ($179 per person round-trip; Sea Planes of Key West, 800-950-2359, www.seaplanesofkeywest.com) or boat ($109 per person; Yankee Fleet, 800-634-0939, www.yankeefreedom.com). As you approach it, the six-sided, three-story fort hovers over the Atlantic like a mirage. In 2003, camping will be available on the beach ($3 per person; 305-242-7700, www.nps.gov/drto).
RIVER OF RUINS TRIP THROUGH MEXICO AND GUATEMALA: This Indiana Jones-style river tour of Mayan ruins begins in Palenque, Mexico. You'll fly to Tikal National Park in Guatemala, and then ride back to Mexico by river on 20- to 80-foot plank boats called lanchas. During your cruise along the Pasion, Petexbatun, and Usumacinta rivers, you'll frequent 1,500-year-old sites like Aguateca and Yaxchilan, accessible only by hiking. A ten-day trip with Ceiba Adventures (800-217-1060, www.ceibaadventures.com) costs $2,550 per person.