Imagine a rainforest that has more species of flora and fauna in several square kilometers than the whole of Europe; a river that carries more fresh water in 1 day than is needed by Central America in an entire year; a nature reserve crisscrossed by rivers that make the jungle a floating gallery of exotic birds and animals; and an archipelago of islands completely peopled by artists inspired by a rebel priest. You don't have to imagine; just go to southeastern Nicaragua.

The Río San Juan floats through a mini-Amazon that is the heart and lungs of Central America. Guarded jealously by the Spanish, attacked by the British, plundered by pirates, and coveted by the Costa Ricans, this area is as rich in history as it is in nature. The psychedelic colors of the jungle are captured in the naïve paintings that emanate from the Solentiname Islands on Lago de Nicaragua, a huge draw for artists, as well as bird-watchers and nature lovers. Nature reserves such as Indio-Maíz and Los Guatazos offer the opportunity to experience a genuine rainforest up close in all its jaw-dropping glory.

Tourists to this area used to be as rare as the jaguar that prowls its vine-draped interior. Now the word is out, and this isolated region is beginning to attract adventurous nature lovers. Lodges are springing up around the river fort of El Castillo, and tour companies offer bird-watching, fishing, and jungle excursions along the river. However, constant rain, muddy paths, and primitive transportation in single-propeller planes and long canoes mean there is no danger of this becoming another Costa Rica. At least, not yet.

This is a mighty river, a broad, majestic, expanse of fresh water that pours slowly out of Lago de Nicaragua towards the Atlantic Ocean 210km (130 miles) away. It passes rainforests and cattle ranches, stilted shacks on the water, and quiet river lodges. It widens in parts to 350m (1,148 ft.), and in other areas, it narrows into treacherous rapids, particularly at the 300-year-old Spanish fort of El Castillo. More than 25 rivers flow into the San Juan from the Nicaraguan and Costa Rican shorelines, and it forms the border between the two countries from El Castillo to the Atlantic estuary. Needless to say, the shoreline teems with wildlife, especially at the mammoth Indio-Maíz Biological Reserve on the Nicaraguan side. The ghost town of Greytown lies at the end, as does its rebranded and relocated sister town San Juan del Norte.