“The Mangareva Islands are about 1,600 kilometres southeast of Tahiti in French Polynesia.”

“They get their name (which means 'floating mountains') from the way the sea spray breaking on the surrounding coral atolls, or motu, causes the ancient volcanic peaks to appear as if they are floating above the waves.”

“Today, the islands are home to about 2,000 people, many of whom work on the pearl farms in the idyllic turquoise lagoon.”

“Dotted across the islands are the remains of dozens of remarkable pieces of architecture: homes built from coral.”

“As part of a larger project studying the transformations of everyday life in 19th-century Mangareva, my archaeology research team has documented dozens of these coral houses, including on the islands of Aukena, Akamaru, Mangareva and Taravai.”

“Now, in a new paper published in the journal Antiquity, we have established the first precise construction timeline for these coral houses.”

“French Catholic missionaries set up an outpost in Mangareva starting in 1834.”

“Traditional buildings of wood and thatch were replaced within a few decades by a new kind of stone cottage.”

“The missionaries often recorded specific dates for their constructions, above all the cathedral in Rikitea, churches throughout the islands, and the main Catholic schools.”

“However, for the largest category of buildings from this time, houses, we usually don’t have any information about construction dates, who built them, and who lived there.”

“We also sampled a branch coral from a pit layer in the same house where I first noticed the 'fresh' looking branches from the coral blocks.”

“Unlike the more well-known radiocarbon dating, where the error ranges are measured in decades, U-Th dates are super precise, narrowing down the date when the corals died, leaving behind the hard exoskeleton, to within a few years.”

“Also unlike radiocarbon, which isn’t very reliable for materials less than about 400 years old, U-Th works right up until the present.”

“An archaeologist visiting Mangareva in the 1930s noted piles of coral rubble he believed were the remains of marae, once sacred structures that were overthrown during the missionary period.”

“There are two potential explanations.”

“Overlapping dates in our U-Th results confirmed this hypothesis.”

“We still have a lot to learn about how people used coral for buildings in the past, and possibly to learn about how coral reefs rebounded, or not, after decades of human exploitation.”

“This might be the more likely scenario, as our 'too old' dates were years or decades, but not centuries, too early. But we also can’t completely rule out the marae theory.”

“We used an advanced technique known as uranium-thorium (U-Th) dating to understand the age of these branch corals – and the structures built from them.”

“We took a 'control' sample from a building with known dates, the 1850s boys’ school from Aukena, as well as samples from an additional eight houses, plus a coral watch tower.”

“Some of the corals apparently died before the 1830s when missionaries arrived. Some even pre-dated European contact in the 1790s.”

“After testing the samples, we were surprised to notice several dates that were older than expected.”

“Another possibility for this kind of coral, from the scientific genus Acropora, is that some branches die off away from the area of active growth on the reef over a period of years or decades but retain their 'fresh' look.”

“During fieldwork in October 2024, I noticed that one of the coral blocks that had fallen from the wall of the ruined house we were excavating had branch corals that looked very fresh, almost like they were just cut from the living reef.”