The Amazon is already a hotbed of archaeological investigation, as researchers find evidence that far more people might have lived in the region than once considered possible. While the Amazon was once thought incapable of supporting large, sophisticated societies, researchers now contend that the region might have been home to thriving urban centers before the arrival of Columbus.

Before those cities were carved out of the forest, people lived in the Amazon’s caves. At Pedra Pintada, a cave that, like those in Carajás, is also in Pará, Anna C. Roosevelt, an American archaeologist, has shown that hunter-gatherers moved to the region 10,900 to 11,200 years ago, far earlier than once thought, about the same time people in North American were hunting mammoths.

Outside the Amazon, remarkable discoveries have been announced in recent months at other Brazilian sites. At Lapa do Santo, a rock shelter near the city of Belo Horizonte, archaeologists said this year that they had found the New World’s oldest known figurative petroglyph. The rock art, a drawing of a man with an oversize phallus, is thought to have been made 10,500 to 12,000 years ago.

Image Work crews throughout the Amazon have found artifacts. Credit... The New York Times

To reach the caves of Carajás, researchers must drive hours along washboard roads cut through the jungle, before scaling escarpments with spectacular views of the Carajás Mountains, a range of canopied peaks rising out of the forest. Macaws fly overhead and bats swirl inside the earth cavities in which hunting tribes once found shelter.

Some of the caves, substantially cooler inside their openings than the surrounding forest, are large enough for more than a dozen people; others might have provided just enough space for two or three people.

Vale, then a state-owned company, began developing the iron ore deposits here after they were discovered in 1967 by a Brazilian geologist on assignment to find manganese for the U.S. Steel Corporation. Vale has since been privatized, but the government still controls big equity stakes.