The largest man-made mound in prehistoric Europe — forty metres of earth raised around 2400 BC. Why is unknown: excavation inside found nothing. In this subject it is known for something else: the fields around Silbury and neighbouring Avebury have produced more crop circles since the 1980s than anywhere else on earth. The figures appear overnight, sometimes a hundred metres across, sometimes mathematically elaborate. In 1991 Doug Bower and Dave Chorley demonstrated how they had been making circles with planks and rope since 1978, and the question of origin largely closed. The circles still appear every summer — made openly now, with some of the makers known by name.

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Montgomery and the Chiles–Whitted Encounter

On the night of 24 July 1948 the crew of an Eastern Air Lines DC-3 was over Alabama. Captain Clarence Chiles and First Officer John Whitted saw a cigar-shaped object with two rows of glowing windows and a tail of flame coming towards them; it passed and climbed away. Both men banked hard to avoid it. One passenger confirmed a flash of light. On the ground that same night a similar object was reported over Robins Field in Georgia. The modern explanation is a large fireball, several of which crossed the south-eastern states in those weeks. The row of windows remains the detail a fireball accounts for least well.