The Rio San Juan forms the southern border between Nicaragua and Costa Rica, flowing into the southwest corners of Lake Nicaragua, and combining river and lake (plus a short stretch of land) to provide the only water passage-way between the Atlantic and the Pacific Oceans in Central America. Historically it was important to the Spanish conquistadors, the British Empire builders, treasure-seeking pirates, and Americans during Gold Rush days.

A 120-mile-long journey down the Rio San Juan takes adventurers on a winding route through the natural wonders of the pristine tropical rainforests of the 8,685-sq.-mile Indio Maiz Reserve, which they share with 600 species of birds and 200 animal species. You’re bound to encounter: howler and spider monkeys, giant anteaters, jaguars (with luck), caiman, turtles and a rainbow of birds (from Chestnut Mandibled toucans to Harpy eagles), flowers and butterflies. The waters of the river have produced some of the world’s largest tarpon, weighing in at over 400 pounds, so naturally, this is prime territory for fishing expeditions.

Boats leave from the town of San Carlos on Lake Nicaragua, and follow the river for 43 miles to El Castillo, a tranquil town that indeed has a 300-year-old Spanish fortress. Short-term explorers stay at river lodges here and take nature tours and birdwatching excursions by boat and on trails within the reserve, go kayaking, take there flashlights for night-touring for caimans. Long-term adventurers take off for the exciting five-day journey to San Juan de Nicaragua on the Caribbean coast.

Getting there:  By boat from San Carlos, 180 miles (290 km) from Managua. There is scheduled air service between San Carlos and the capital.