Neolithic engineers may have used ball bearings in the construction of Stonehenge in Wiltshire, claim experts from the University of Exeter.
Scientists showed how balls placed in grooved wooden tracks would have allowed the easy movement of stones weighing many tonnes.
Nobody has yet explained successfully how the heavy slabs used to build Stonehenge more than 4,000 years ago were shifted from their quarries to Salisbury Plain.
Some, the "bluestones", weighed four tonnes each and were brought a distance of 150 miles from Pembrokeshire, Wales.
Attempts to re-enact the transporting of the blocks on wooden rollers or floating them on the sea have not proved convincing. The hard surfaces and trenches needed when using rollers would have left marks on the landscape, but none is there.
Experts hit on their new theory after examining mysterious stone balls found near similar monuments in Aberdeenshire, Scotland. About the size of a cricket ball, they are precisely fashioned to be within a millimetre of the same size, suggesting they were meant to be used together rather than individually.