Silbury Hill
Rising 130 feet and comparable in height and volume to the roughly contemporaneous Egyptian pyramids, Europe's largest man-made mound was built around 2400 BC. Though there have been periodic excavations of the mound since the 17th century, its original purpose remains unknown. The viewing area, less than 1 mile east of Avebury, is open only during daylight hours, but there's no direct access to the mound itself.