Picture 1 of 2.
Trethevy Quoit on Bodmin Moor is around 5,5000 years old (dating back to the Neolithic period, 3500 BC). The box shape of stones was originally the centre of a mound, probably holding a burial, over time the mound has worn away.
This picture showns the sun through a hole in the roof of the structure.
Apologies to jamesrb and Annamaria - I used the wrong copy of this picture so when I changed it I lost your comments.