Enigmatic canal-filled ruins may have been above water when built

Even by the standards of enigmatic ancient ruins, Nan Madol is strange. Constructed primarily of "logs" made of volcanic rock, the site consists of dozens of small artificial islands separated by canals that are flushed by the tides. It's built on the shores of the Micronesian island of Pohnpei, which has an enigmatic history, seemingly remaining unpopulated as islands to the north and south were settled during the Polynesian expansion.

Now, a team of researchers is offering a single explanation that accounts for many of these oddities: The island of Pohnpei is slowly sinking, taking evidence of an earlier settlement beneath the waves. And, if their estimate of its subsidence is accurate, Nan Madol would have been above the waves at the time of its construction.

Ups and downs

The human expansion into the scattered islands of the Pacific started over 3,000 years ago and primarily took place upon two parallel routes north and south of the equator. The southern route was peopled by the ancestors of the Polynesians, while the northern route was derived from people who likely originated in the Philippines. There were islands between the two along the equator, but those weren't settled until roughly a thousand years later when the descendants of the first wave expanded from the islands they had initially populated.

One explanation for this involves changes in sea level. Many of the islands between the northern and southern expansions are low-lying atolls, which could have been entirely underwater at that time. That's because sea levels across the equatorial Pacific were higher as the Earth's crust adjusted to the redistribution of water away from massive ice sheets of the last glacial period. In this view, ocean levels in this region gradually dropped, revealing more of these atolls and making them easier to settle.

That doesn't explain everything, however. Pohnpei and Kosrae islands are in the region, and they're both centered on volcanic peaks that would have extended well above sea level the entire time. Yet there's no indication they were settled before other islands in this region. Roughly 3,000 years after their settlement, a new culture arrived and constructed substantial urban centers: Nan Madol on Pohnpei and Leluh on Kosrae.

Both sites feature similar construction. Large basalt columns, like the ones found at the Giant's Causeway or Devils Tower, are arranged a bit like the logs on a log cabin. Blocks of coral are also used. While Leluh was built on a small island off the coast of Kosrae, Nan Madol is in the water, with buildings separated by canals, giving it the Venice of the Pacific nickname. While the materials and architecture are similar to each other and shared with other sites in the Pacific, the canals are unique to Nan Madol.

Rising tides

The new work started with a look at sediments formed in mangrove swamps. Mangroves only grow within a limited range relative to the high tide mark of the area, and will trap sediment in that region. If tide levels are stable, that limits sediment formation to a range of less than a meter around the high-tide mark. Working with sediments on these islands, however, the research team found mangrove sediments up to six meters thick in areas around the islands. This implies that the islands have been steadily sinking, allowing sediments to build up on top of previous layers.