It was a lost Spanish conquistador who gave a name to the largest river and rainforest on earth. In 1541 Francisco de Orellana returned after a year's absence with a tale of a vast new river, and of attacks by hostile Indians, some led by bands of female warriors. In Europe, these women warriors were taken to be the last remaining members of the tribe whose queen Achilles slew at Troy. Land and river both were named for this likely mythical tribe -- the Amazons.

Three centuries later, it was a British explorer, Alfred Russel Wallace, who first noted what is today very likely the Amazon's greatest claim to fame -- its diversity. The Amazon rainforest has more biodiversity -- more species of plants and animals per given patch of forest -- than any ecosystem on earth. Being a naturalist, Wallace also proposed an explanation. It was due, he believed, to the Amazon's extraordinary number of rivers, streams, and channels, which effectively cut the forest up into millions of small islands. Marooned and isolated, a separated single species would evolve into many new species.

So logical did this seem that only in the past 10 years did scientists try to test it out. At which point they discovered it, too, was a myth. The reason for the Amazon's diversity has once again retreated into mystery.

The Brazilian Amazon today remains one of the most isolated, most sparsely populated regions on earth. Yet it is also home to more than four million people. Tarzan myths aside, most of them live not in the jungle, but in cities. Indeed, the state of Amazonas is one of the most urbanized in Brazil.

The Amazon's two principal cities are Manaus and Belém, capitals of the states of Amazonas and Pará, respectively. Belém sits at the mouth of the river, Manaus at the spot where the two principal tributaries -- the Rio Negro and the Solimões -- join to form the Amazon. Belém is the older, more civilized of the two, with colonial architecture, forts, stately churches, and a sense of itself as the natural senior partner, somehow left behind by a brash new upstart. The upstart, Manaus, has in the past 50 years grown from a city of a few hundred thousand to a metropolis of almost two million. There's an exciting, frontier feel to Manaus. Belém offers history, excellent food, and much better nightlife. Manaus offers a sense of its own future and an opportunity to step into the vastness of the tropical rainforest.

A visit to the Amazon is a chance to penetrate the mystery, to experience the tangle of myth and reality firsthand.