Silbury Hill is a prehistoric artificial chalk mound near Avebury in the English county of Wiltshire. It is part of the Stonehenge, Avebury and Associated Sites UNESCO World Heritage Site.
At 30 metres (98 ft) high, Silbury Hill – which is part of the complex of Neolithic monuments around Avebury, which includes the Avebury Ring and West Kennet Long Barrow – is the tallest prehistoric human-made mound in Europe and one of the largest in the world; it is similar in size to some of the smaller Egyptian pyramids of the Giza Necropolis. Its original purpose is still highly debated. Several other important Neolithic monuments in Wiltshire in the care of English Heritage, including the large henges at Marden and Stonehenge, may be culturally or functionally related to Avebury and Silbury.
It's MASSIVE about 4750 years old and we aren't sure why so many people spent so much time building it or what's it's purpose was.