The mysterious aqueduct-filled ruins may have been above water when it was built

Zoom in / Distinctive ruins at Nan Madol.

Even by the standards of a mysterious ancient ruin, Nan Madol is strange. Constructed mainly of “logs” made of volcanic rock, the site consists of dozens of small artificial islands separated by channels washed by the tides. It was built on the shores of the Micronesian island of Pohnpei, which has a mysterious history, and appears to have remained uninhabited as islands to the north and south were settled during the Polynesian expansion.

Now, a team of researchers offers one explanation that explains many of these oddities: Pohnpei Island is slowly sinking, taking evidence of earlier settlement beneath the waves. And if their estimate of its descent is accurate, Nan Madol would have been above the waves at the time of its construction.

Ups and downs

Human expansion began on scattered Pacific islands more than 3,000 years ago and occurred mainly along two parallel routes north and south of the equator. The southern route was inhabited 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 were not settled until nearly a millennium later when the descendants of the first wave expanded from the islands they initially inhabited.

One explanation for this involves changes in sea level. Many of the islands between the northern and southern expansions are low-lying atolls, which would have been entirely underwater at the time. That’s because sea levels across the tropical Pacific were higher as the crust adapted to redistribute water away from the massive ice sheets of the last glacial period. From this point of view, the ocean levels in this region have gradually decreased, exposing more of these atolls and making their settlement easier.

But this does not explain everything. The islands of Pohnpei and Kosrae lie in the area, both of which are centered on volcanic peaks that would have extended above sea level all along. However, there is no indication that they settled before the other islands in this region. Nearly 3,000 years after their settlement, a new culture arrived and built great urban centers: Nan Madol in Pohnpei and Liloh in Kosrae.

Both sites feature a similar architecture. Large basalt columns, such as those at Devil’s Causeway or Devils Tower, are arranged somewhat like logs in a log cabin. Blocks of coral are also used. While Leluh is built on a small island off the coast of Kosrae, Nan Madol is set in the water, with buildings separated by canals, giving it the nickname Venice of the Pacific. While the materials and architecture are similar to each other and in common with other Pacific sites, the canals are unique to Nan Madol.

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The new work begins with a look at sediments formed in mangrove swamps. Mangroves only grow to a limited extent relative to the high tide mark in the area, and will trap sediment in that area. If tidal levels are stable, this limits sediment formation to a range of less than a meter around the high tide mark. However, working with sediments on these islands, the research team found mangrove deposits up to six meters thick in the areas around the islands. This means that the islands were steadily sinking, allowing sediment to build up on top of the earlier layers.

Tracking these sediments at multiple locations, along with carbon dating of the materials in the sediments, has allowed the researchers to reconstruct sea level over the past 5,000 years or so. These show that the reflux is likely to recede for most of that period. So, while the ocean level in the area may have been dropping for part of this period, the local sea level was actually rising as the islands slowly slipped into the sea. The GPS station at Leluh indicates that the descent has continued to the present.

(The decrease may be related to the fact that these islands are products of hotspot volcanism, propelled by a plume of hot material in the mantle. But the Pacific Plate has now moved so that the hotspot is no longer beneath these two islands.)

Overall, the researchers concluded that the two islands could have been settled during the initial expansion into the Pacific Ocean. But if people stayed on the coast as they did on many other islands, the remains of the first settlements would be underwater at present. People will no doubt move inland more for things like farming, but these activities may not have left anything that could survive as an artifact.

As for Nan Madol, reconstructions of sea levels indicate that they were about a meter lower at the site when construction began. This would have left the ‘channels’ completely dry except at very high tide. Except during storms, the worst flood would have been only a few centimeters. Even several hundred years later, when the rulers who built Nan Madol were depopulated and the site abandoned, the canals would have dried up at low tide. Therefore, they probably shouldn’t be seen as channels at all.

PNAS2022. DOI: 10.1073/pnas.2210863119 (about DOIs).

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