GeoGraph - photograph every grid square

Deep pothole on the slopes of Eldon Hill near Castleton. According to the booklet Caves and Karst of the Peak District (BCRA Cave Studies Series Number 3): "The Hole forms a gaping chasm, 55m deep on the south flank of Eldon Hill. A tight, loose drop at the bottom leads into a large chamber and an 18th-century account suggests that there is a second drop in the floor to an underground river but it has long been concealed by boulders thrown down." See Link for more information.