When it was built in the 4th century, the Jetavana Buddhist shrine in the holy city of Anuradhapura was one of the architectural marvels of the age. Standing 400 feet high, it was the third tallest structure of its day behind the Pryramids of Giza.

Now its domed top has fallen off and a mound of tree-covered earth hides the height that made it so special more than 1,500 years ago. Nonetheless, it remains the world's tallest stupa, or domed Buddhist shrine, and still is a center of religious worship on this predominantly Buddhist island.

The Sri Lankan government, along with UNESCO, has begun a $19 million project to rehabilitate this country's most sacred and beautiful Buddhist relics, which are contained in a triangle formed by the cities of Anuradhapura, Polonnaruwa and Kandy.

This is UNESCO's 19th project to preserve some of the endangered footprints of the world's distant past, including the Acropolis in Athens and temples in Egypt that would have been flooded by dam construction.

"These monuments don't belong to a country or a region, but to the entire world," said Roland Silva, the Sri Lankan architect who is deputy commissioner of archeology here and director general of UNESCO's Sri Lankan project.

Each of the three cities forming Sri Lanka's "cultural triangle" has something special to offer. Anuradhapura, the first capital of Buddhism since it was introduced to this island around 300 B.C., contains imposing stupas, monasteries, artificial lakes and pleasure gardens. Polonnaruwa has been described as a fairy-tale city, with its three concentric walls forming a fortification. It is dominated by Galvihara, a massive statue of Buddha. Kandy, the third city, is the home of the Temple of the Buddha's Sacred Tooth, the center of worship here.

"All these treasures are now in jeopardy," said Amadou-Mahtar M'Bow, UNESCO director general.

"The combined action of climate, fauna and flora has slowly taken its toll. Rain and wind have insidiously corroded the stonework; the action of savage vegetation and certain animals, in their turn, have addtionally eaten into the monuments."

The 400-foot-high Jetavana stupa and its older and shorter neighbor in Anuradhapura, the Abhayagiri stupa, are two of the most extensive and challenging restorations in the UNESCO program.

Only 83 feet shorter than the Pyramids at Giza, the Jetavana stupa is amazing in that craftsmen of this small Indian Ocean island appear to have matched the skills available in such major nation-states of the time as Egypt, Greece and Rome, Silva said.

Both the Jetavana stupa and the 380-foot-high, 2nd-century B.C. Abhayagiri stupa, the second tallest Buddhist shrine in the world, are covered by dense overgrowth that hides both their size and the subtle beauty of their design.

It is almost impossible to remove the earth mounds that cover the stupas' bases, since the roots of trees and shrubs have pushed into the mortar and are in fact holding the structures together. Instead of trying to cut down the underbrush and trees and remove the mounds, restorers have decided to trim the bushes and trees to give the mounds a neat appearance and concentrate on rebuilding the gardens and monasteries that once surrounded the stupas.

Another major project is the restoration of the 5th-century water gardens at Sigiriya just west of Polonnaruvwa. These gardens, designed by Kasapa, the poet-king of Sigiriya, reflect an unparalleled sense of garden architecture for their time, said Silva.

"It's a marriage of the formal Renaissance garden and Oriental-Japanese landscaping, both playing alongside each other," he continued.

The Sri Lankan government is seeking the help of a University of Maryland expert, Prof. Wilhelmena Jashemski, in restoring the Sigiriya gardens.

UNESCO contributes only seed money to start a project; now Sri Lanka must find the funds for the actual reconstruction. The country is asking the World Food Program to contribute surplus food to be given in lieu of cash to laborers on the archeological digs required for four of the six projects. The government is also looking to other countries and foreign universities for contributions.