Rising sea levels are damaging one of the ancient world's greatest building projects--a series of artificial islands in Micronesia known as Nan Madol. According to University of Oregon archaeologist William Ayres, the residents of the island of Pohnpei first piled boulders on a barely submerged coral reef about 2,000 years ago, creating small islands for houses that, over the centuries, grew to include temples, tombs, royal residences, and homes for up to 1,000 people. When the site was conquered and subsequently abandoned in the 1600s, it covered more than 120 acres. Since 1977, Ayres has seen pounding waves and increasingly powerful tides wash away the sand that stabilizes the islands. "Once the islands begin to deflate, the cultural remains shift, and you lose a lot of information," he says. "Then the foundations begin to shift and the exterior walls of the islands collapse."