Neolithic HS2?

An artist's impression of the building of Silbury Hill.  Apparently it took place over centuries; as it grew, the ditches from which material had been taken for the construction were in the way and had to be backfilled.  That's planning ahead for you.

As to why, no one knows.  Presumably life was so good and easy back then that everybody had loads of spare time to dig up earth with deer scapulae and just pile it up.  Or maybe they had a wacky leader in charge.