View of Stonehenge, England, as it appears today. Stonehenge was built in various stages c. 3100 - 1000 BC. The original structure consisted of three circles of stones arranged in such a way that the outer ring had 56 openings and the inner two had 30 and 29 openings. It allowed observations of star settings at various locations.
More than 600 such structures - not all as elaborate as Stonehenge - were built in Britain and mainland Europe (mainly in Brittany). Ruins of many of these can still be seen today.
Photo: © F. Vincent; Creative Commons Attribution ShareAlike license (Wikipedia)