Nearly two-thirds of Peru is Amazon rainforest, which thrives with some of the richest biodiversity on the planet. Covering 6,475,000 sq. km (2,500,000 sq. miles), the Amazon basin represents 54% of all remaining rainforest on the planet. This vast, largely impenetrable region, with the smallest human population in the country and few towns of any significant size, clearly is not the Peru of great pre-Columbian civilizations and Inca ruins. It stands in stunning contrast to the country's rugged Andean peaks and arid desert coasts. The humid frontier towns of the jungle, well past stages of oil and rubber booms and now hell-bent on ecotourism, are worlds removed from the historic cities Cusco and Arequipa and the modern madness of Lima.

Many naturalists and biologists believe that Peru's Amazon rainforest holds the greatest diversity in the world. It teems with a staggering roster of wildlife: 400 species of mammals, 2,000 species of fish, 300 reptiles, 1,800 birds, and more than 50,000 plants. Recent studies have shown that a region just south of Iquitos has the highest concentration of mammals anywhere in the world.

Not surprisingly, jungle ecotourism has exploded in Peru, as it has in several other Latin American countries. Peru's jungle regions are now much more accessible than they once were -- which is both a good and a bad thing, of course -- and there are more lodges and eco-options than ever. Remote as the Amazon jungle surely seems, it is possible to find yourself at an ecolodge in as little as 12 hours after boarding a flight in the U.S. Still, accessibility is a crucial factor in jungle trips: The more remote a lodge or camping trek is, and the more pristine and unspoiled the environment is, the more it's going to cost you to get there in terms of both time and money. Rivers define life in the jungle even more than do the forests; for both locals and visitors, almost all transport along the vast river system that stretches across the whole of eastern Peru is by dugout canoe, motorboat, or large riverboats (lanchas).

The southern Amazon region, which extends to the Bolivian and Brazilian borders, is concentrated in the Madre de Dios department, the least populated area in Peru. Although it is accessible by land from Cusco, it is an exceedingly difficult route. Most travelers fly to Puerto Maldonado (the gateway to the Tambopata National Reserve) and travel overland to the Manu Biosphere Reserve, returning by small aircraft.

The northern Amazon reaches all the way to Peru's borders with Colombia and Brazil. The gateway to Peru's northern Amazon basin is Iquitos. As an example of how huge the Amazon is, Iquitos lies nearly 3,220km (2,000 miles) from the mouth of the great Río Amazonas, the second-longest river in the world. Other than an arduous journey by boat, the only way to get to Iquitos is by airplane (usually from Lima).

The best time to visit the Amazon is during the dry season, May through the end of October. During the rainy season in the southern Amazon, parts of the jungle are flooded and impassable. The northern jungle does not have a rainy season, per se, and travel there is less restricted during the winter. However, water levels can rise from 7.5m (25 ft.) to more than 15m (50 ft.) from December to May, and some jungle villages become flooded. Many naturalists find high-water months best for wildlife observation.