For thousands of years before Europeans "discovered" South America, great cities and nations rose and fell on that continent, we guess.

We guess because we don't know much except the broad outlines. The pre-Columbian peoples were slain or enslaved and their treasures looted by the conquistadors and those who followed in their train. Long after the gold and silver and jewels were gone, the looters, some acting in the name of science, went on "collecting" the great statuary, superb pottery and other artifacts of those destroyed cultures.

It would be a sad-enough tale told as history, but the looting's still going on, as a new exhibit of Peruvian national treasures at the National Geographic Society reminds us. For centuries the remoteness of Peru's high desert limited the rape of that nation's ancient ruins, but modern transportation and fabulous prices have resulted in an "artifacts rush" that has left hundreds of miles of countryside scarred by tomb-robbers and site-seekers.

The tragedy is not just, or even principally, the loss of the objects but the loss of the contexts in which they were made and used. Because there was no written language, the cultures must be puzzled out by painstaking excavation, recording the development of techniques and alterations of style, subtle changes in the patterns of dwelling and temple layout, kitchen middens, spilled seeds and discarded fruitpits, all the mundane -- and crucial -- details that are obliterated by furtive diggers.

A 1981 agreement between the U.S. and Peru now classifies American dealers in Peruvian antiquities as receivers of stolen goods, subject to search, seizure and prosecution. A nation that can't keep shiploads of marijuana from being landed on its shores is hardly going to halt artifact-smuggling, but the humiliating arrests of just a couple of genteel dealers is said to have had a wonderfully depressing effect on the market. The 500 objects on display are among 800 seized by customs inspectors so far.

The problem and what's being done about it are well laid out in the exhibit, particularly the central section that calls the dolorous roll of Peru's vanished cultures: Chavin, Paracas, Nazca, Moche, Tiahuanaco-Huari, Chimu, Inca . . . brief as it is, it tells more of ancient South American history than most Americans will have been exposed to.

The exhibit misses a good bet, though, in mishandling the one item that might have appealed to young people. A videotape called "The Owl: A Fable," in which a pottery shard mourns its loss of identity, has arresting graphics but highfalutin language. STOLEN TREASURES, MISSING LINKS -- Through December in Explorers Hall, National Geographic Society headquarters, 17th and M streets NW. Open 9 to 5 Saturdays and holidays, 10 to 5 Sundays, 9 to 6 weekdays.