What is "denehole"

A denehole (alternatively dene hole or dene-hole) is an underground structure consisting of a number of small chalk caves entered by a vertical shaft. The name is given to certain caves or excavations in England, which have been popularly supposed to be due to the Danes or some other of the early northern invaders of the country. The common spelling Dane hole is adduced as evidence of this, and individual names, such as Vortigerns Caves at Margate, and Canutes Gold Mine near Bexley, naturally follow the same theory. The word, however, is probably derived from the Anglo Saxon den, a hole or valley. The lack of evidence found in them has led to long arguments as to their function.