Neolithic people in Great Britain were really into building big things; the British Isles are studded with ancient hill forts, monumental graves and ritual sites. Now, a new study suggests another type of landmark needs to be added to the list—artificial islands ranging from 30 to 100 feet in diameter, called crannogs.

Crannogs are found off the banks of rivers and lakes throughout Ireland and Scotland. According to the Scottish Crannog Centre, the structures were originally round homes built over the water, either by pounding piles into the muck or by moving tons of rock and dirt to create an artificial island. Today, archaeologists have catalogued the remains of hundreds of these waterside islands, most of which now look like tiny, tree-covered islands or mounds that skirt just below the water's surface.

Erin Blakemore at National Geographic reports that archaeologists originally believed that most crannogs dated to the Iron Age of the British Isles, from about 800 B.C. to 43 A.D. But the new study in the journal Antiquity has found artifacts placing the mounds back much more, to around 3500 B.C., roughly the time of the Henge builders.

Researchers had an inkling that some crannogs had Stone Age beginning in the 1980s, when excavations at a crannog at Scotland’s North Uist island revealed Neolithic artifacts. But in the ensuing decades, digs at other sites didn’t come up with more evidence. Then in 2011, Bob Yirka at Phys.org reports, a former Royal Navy diver was exploring the waters around one of the crannogs on the Isle of Lewis in the Outer Hebrides, a remote chain of islands off the west coast of Scotland, when he found some interesting ceramics. After bringing the piece to a local conservation officer at a museum, the pair explored the area more, finding similar Neolithic ceramics around other local crannogs.

Later, study authors Duncan Garrow of the University of Reading and Fraser Sturt of the University of Southampton took over, radiocarbon-dating pot residue and timbers from the crannogs. Harry Cockburn at The Independent reports that six of eight timbers dated were from the Stone Age, dating to 3360 to 3640 B.C.

So why were Neolithic people investing the time and resources to hauling stones, some up to 550 pounds, into the water to build these monumental islets and, in one case, a stone causeway in Loch Bhorgastail? It’s hard to say, but there is one clue. Many of the ceramic vessels found near the sites are remarkably intact, likely submerged in one piece and then protected for 5,000 years by placid waters and deep sediment. In comparison, most pottery of this age is found broken into shards at other sites. The inside and outsides of the ceramics also show signs of charring, meaning they were used for some purpose. “I’ve never seen anything like it in British archaeology,” Garrow tells Blakemore. “People seem to have been chucking this stuff in the water.”

It’s possible that the islets were sites for religious or funeral rituals or for feasting. Vicki Cummings, an expert on the Neolithic from the University of Central Lancashire, not involved in the study, tells Blakemore that because the crannogs were built away from any known villages or settlements and distance from tombs or burials suggests they were for other rituals, perhaps coming of age ceremonies. “These islets could also have been perceived as special places, their watery surroundings creating separation from everyday life,” the authors write in the paper.

Getting answers means looking deeper at these islets and dating more of the 600 known Scottish crannogs, 80 percent of which have not yet been tested. It’s also possible that the Lewis Isle sites are unique and other crannogs are much younger. The team hopes to use sonar to identify more hidden crannogs in the Outer Hebrides. And they want archaeologists to revisit crannogs dated to the Iron Age or Medieval Period to see if they’re built on Neolithic foundations.

Whatever their purpose in the Neolithic, by the Iron Age, the crannogs became dwelling sites where generations of people lived. Archaeologists are still trying to figure out why anyone chose to live on the tiny islands, though William Butler Yeats may have had a good idea.