Some go mainly for adventure, others for knowledge, fame or wealth - just like their 16th-century counterparts.

All are hooked on the sweet mysteries suggested by the explorers' poet laureate, Rudyard Kipling: "Something hidden. Go and find it . . . Something lost behind the Ranges. Lost and waiting for you. Go!""A lot of people, especially in the United States, think every square foot of the world is known and mapped - except maybe the bottom of the sea," professional trekker David H. Childress told National Geographic. "That's just not so."

Standing on a dusty road near the outskirts of Cuzco, the old capital of the Incas and now a reasonably modern city of about 140,000 people, Childress points toward the pale blue mountains to the northeast.

"A hundred miles from here is jungle that is completely unexplored, inhabited by Indians who would kill you on sight," he says.

Some of these local residents know the location of "lost cities" - the remains of civilizations that have otherwise vanished from the earth. Sometimes a friendly native will guide you to them, as happened to Yale historian Hiram Bingham in 1911, when an Indian farmer living along the Urubamba River valley first showed him the fabulous Inca ruins at Machu Picchu.

Satellites and remote-sensing techniques notwithstanding, some things haven't changed much since Bingham's day.

"It's amazing that the descriptions Bingham made of these places are exactly as they are now," says anthropologist and Andean scholar Johan Reinhard. "You still have to get people to carry your things or get meals to a certain point and then go on from there. His descriptions of certain villages have hardly changed at all."

Reinhard and others say the landscape of the Andes is littered with ruins waiting to be discovered. Present-day explorers as diverse as Reinhard, a Ph.D. research associate with Chicago's Field Museum, and Childress, a college dropout, are still hunting them.

"One of the most famous sites in Western Hemisphere archaeology is in the Cordillera Blanca of Peru," says Reinhard. "I was doing some research there recently and I started climbing the surrounding hills. I just found ruins all over the place that nobody had reported: no terribly important cities, but 50 or 60 structures with walls up to 2 meters high in places.

"A tremendous amount is left to be discovered. It's just a matter of going out and looking."

Some of the searching is now done from the air. Two years ago, Reynaldo Chohfi, a California graduate student, hacked his way through a Peruvian jungle to a previously unknown city about twice the size of Machu Picchu. Chohfi knew it was there. He had seen it in an aerial photograph.

People have been looking for lost civilizations in South America ever since 16th-century Spanish adventurers equipped with armor and harquebuses sought the legendary city of El Dorado and its reputed hoard of gold. More recently, Nevada-based explorer Gene Savoy, who has been crisscrossing Peru since 1957, has assembled expeditions using helicopters, twin-engine airplanes, field laboratories and canoes with outboard motors.

So far no one has found artifacts from El Dorado, but in 1985 Savoy reported finding a pre-Columbian metropolis about 400 miles northeast of Lima. It contained almost 24,000 structures, now inhabited only by monkeys.

As always, traveling in unmapped jungle areas can be hazardous. Some explorers have vanished. In 1925, for example, retired British army engineer Percy Fawcett disappeared in Brazil's Mato Grosso while he was searching for El Dorado.

"The danger thing can be overplayed," says Reinhard. "But it's always dangerous when you go out into unexplored areas. You could step on a viper or cut yourself with a machete, or get caught in an area where going back is worse than going forward. You're kind of lost, you're running out of water and you have to cut your way out. Stuff like that."

Modern explorers have other discomforts to contend with, including heat, tropical illnesses, piranha and strange insects that can inflict terrible tortures. And, the prospect of finding hidden treasure aside, the work is not especially lucrative.

Childress, 28, supports himself by organizing jungle treks and publishing, at his own expense, books about his adventures. Reinhard, 44, has received scientific grants and awards and sometimes lectures tourists aboard cruise ships.

"It's not as great a life as it may sound," Reinhard concedes. "I'm not married, no children, no home, not a dollar in a retirement plan. Not much security."

He recounts a legend that after the Spaniards killed the Inca Atahualpa, his people took all the gold they had been collecting for his ransom and threw it into an artificial lake.

Reinhard thinks he knows where the lake is. It's somewhere in the ranges of southern Peru, he believes, waiting . . .