For centuries after Portugal colonized Brazil, the Amazon rainforest was nicknamed the Green Hell, an impenetrable rain-sodden jungle that offered only danger to outsiders. Many expeditions, above all those hoping to find gold in the mythical lost city of El Dorado, never returned. Some may have been killed by hostile native tribes or died of snake bites or starvation, but a surprisingly large number of explorers chose to settle down with indigenous tribes and share their bucolic way of life. Today, the rainforest is viewed more benignly, sometimes even romantically, as the Green Paradise, more often simply as an extraordinary treasure of nature, one with the planet's largest concentration of botanical species, including some 16,000 species of trees and innumerable plants with remarkable medicinal potential. Further, this unparalleled concentration of vegetation absorbs greenhouse gases and breathes out oxygen. It also offers a traditionally secure home to hundreds of indigenous tribes, some of whom have never had contact with the outside world.