Silbury Hill: A detailed article, linked below on how the hole in the top of this iconic monument was discovered 25 years ago, and later repaired at great expense. Don't climb the hill. The largest prehistoric human-made mound in Europe, Silbury Hill is 39m (128ft) high, covers an area of 2.2 ha (5.5 acres) and is estimated to have taken 18 million working hours to construct. 📷 Pete Glastonbury More: