Anhydrite (Calcium Sulphate) was mined at Billingham from 1927 until 1971 when it became unprofitable to carry on. In the 44 years the mine operated over 33 million tons were extracted and used principally to make fertilizers and cement. The Room and Pillar method of mining was employed, with an extraction efficiency of about 50%, which means that half the Anhydrite was left unmined.
This method, coupled with the natural strength of the rock, gives massive stability to the mines structure, and prevents any subsidence. In 1979 the shafts were capped off. The mine extends over a distance of about a mile east to west and two miles north to south. About two-thirds of the underground workings are beneath ICI owned land.
The remainder lies underneath farmland, industrial development and housing. Access is via two main shafts which are inside the Billingham factory. Nothing is stored down the mine, which is assumed to have remained dry since it was capped off.