In 2004, Wren's Nest and the nearby Castle Hill were declared Scheduled Ancient Monuments, as they represented the best surviving remains of the limestone industry in Dudley. The most impressive part of this is the last remaining surface opening limestone cavern in the world – formerly reaching more than 100 metres underground – which is known as the Seven Sisters. The workings were originally connected by underground canal to the Dudley Tunnel complex, which has now been blocked off for safety reasons.