Stonehenge is a prehistoric monument on Salisbury Plain in Wiltshire, about 2 miles west of Amesbury. An outer ring of sarsen stones about 4 meters tall, capped by lintels, surrounds smaller bluestones and free-standing trilithons. It was built in phases, beginning with a circular ditch and bank around 3100 BC; the sarsen circle went up between 2600 and 2400 BC. The sarsens have been traced to West Woods, about 16 miles north, most bluestones to the Preseli Hills in Wales, and, in a 2024 study, the Altar Stone to northeast Scotland. The monument is aligned on the summer solstice sunrise and winter solstice sunset, is owned by the Crown and managed by English Heritage, and has been a UNESCO World Heritage Site with nearby Avebury since 1986.

Its builders left no written records, and explanations multiplied. Geoffrey of Monmouth credited Merlin, 18th-century antiquarians linked it to Druids, and in the 1960s astronomer Gerald Hawkins argued that it was laid out to track the Sun and Moon; archaeologists have since proposed a burial ground and a place of healing. UFO lore entered with the ley-line revival. In 1961 Tony Wedd argued that alignments of ancient sites guided alien spacecraft, and John Michell spread the idea in his 1967 book The Flying Saucer Vision. Ancient-astronaut writers also list Stonehenge among monuments they say could not have been built with the tools of the time. Archaeologists regard ley lines as pseudoarchaeology and reject the ancient-astronaut claims.

The surrounding fields added crop circles. Researcher Jeremy Northcote found that British formations cluster near roads, towns and monuments such as Stonehenge and Avebury, a pattern pointing to human makers. On July 7, 1996, a fractal formation known as the Julia Set appeared in a field near the stones; enthusiasts cite a pilot's account that it was absent on one pass and present about 45 minutes later, in daylight. French researcher Patrick Gross found the witness accounts conflicting and unverified, and circle makers Rod Dickinson and John Lundberg have been reported as saying they made it.

In 2026 a study in the Journal of Quaternary Science concluded that, whatever part glaciers played, people deliberately carried the Altar Stone on the final leg of its journey from Scotland. On American Alchemy, Geoffrey Drumm argued that standing stones such as Stonehenge's worked as lightning rods to fertilize the land, an idea outside mainstream archaeology. Stonehenge remains the emblem of claims that ancient builders had outside help, even as excavation keeps adding detail about the people who raised it.