COASTAL DUNES, LENÇÓIS MARANHENSES, BRAZIL

"It feels like a parallel world," says Carolina Alvite, former director of the 600-square-mile national park created three decades ago to protect this unlikely ecosystem. It's as if the sea near the Bahamas suddenly appeared like a mirage in the middle of the Sahara.

Only in this desert the mirage is real.

Actually, by the most technical of standards, the Lençóis isn't really a desert, says Antonio Cordeiro Feitosa, a geographer at the Federal University of Maranhão. Forty-seven inches or so of rain fall on the region a year. By definition, a desert averages fewer than ten inches a year.

And it is the presence of water that makes the sandscape possible. Two nearby rivers, the Parnaíba and Preguiças, carry sand from the interior of the continent to the Atlantic, where ocean currents push them westward. Much of the sediment is deposited along the park's 44-mile-long shoreline. Here, during the dry season, especially in October and November, a relentless northeasterly wind drives the sand as far as 30 miles inland, sculpting crescent-shaped dunes up to 130 feet high as far as the eye can see. Cordeiro's eye also sees the perpetual motion of Lençóis Maranhenses. In places these dunes may advance by as much as 65 feet a year. "The landscape is radically transformed by each seasonal cycle," he says.

Lagoons are born anew each year after the January-to-June rains fill the valleys between the dunes. Some of these temporary bodies of water are more than 300 feet long and up to ten feet deep. In early July, at their fullest, the lagoons can become interconnected when rivers, such as the Rio Negro, cut through the dunes. So fish have a route to migrate into the lagoons, where they feed on other fish or the larvae of insects buried in the sand. A few fish species, such as the wolffish (Hoplias malabaricus), even spend the dry season dormant in the mud, emerging when the showers return. Once the rainy season ends, the lagoons begin to evaporate in the equatorial heat; their water level may drop by as much as three feet a month.

Fish and bugs aren't the only residents. Besides people living in villages around the dunes, 90 men, women, and children inhabit two oases in the dunes, Queimada dos Britos and Baixa Grande, where they dwell in palm-roofed mud huts. Like the dunes, they change their routines with the seasons. In the dry season they raise chickens, goats, and cattle, grow cassava, beans, and cashews, and collect buriti and carnauba palm fibers from the restinga, or coastal vegetation, that grows near the dunes. When the rains come and planting becomes more difficult, people in the oases head for the sea, where they live in fishing camps on the beach. There they sell salted and dried Atlantic tarpon and other fish to traders, who sell them in the cities.