Rocky outposts are much older than previously supposed and might have served ritual purposes.

Four engineered islands rising from Scottish lochs are more than 5,000 years old — a testament to the sweat and organizational skills of the early farmers who hauled tonnes of rock to build the structures.

Lakes and inlets in Scotland and Ireland are dotted with hundreds of purpose-built islets called crannogs. Scientists had long thought that almost all of these crannogs were erected after 800 BC.

Duncan Garrow at the University of Reading, UK, and Fraser Sturt at the University of Southampton, UK, studied pottery found in the waters around crannogs in the Outer Hebrides, a chain of islands off the Scottish coast, and surveyed two of the crannogs in detail. The researchers dated four crannogs to 3640–3360 BC. One crannog measures 26 metres by 22 metres and includes rocks weighing as much as 250 kilograms apiece.

The outposts’ purpose is unknown, but they may have served as sites for feasts and other rituals.