Deep below the ground by a farm in South Mimms is a series of huge caves, the home to colonies of bats. They were discovered over thirty years ago by a caver, John Huston, and are only accessible from three vertical shafts, although once there must have been a larger shaft wide enough for horses and carts. They are actually a vast chalk mine, and were in use during the 1800s to supply calcium carbonate and flint to the building trade.

They remain virtually unknown to local people, but with 1,132 ft of tunnel known, up to 40 ft high and 32 ft wide, they are one of the largest unsupported chalk mines anywhere in the world. The miners were expert at excavating the caves, creating vaulted structures as they went deeper to provide stability. They provided some 23,000 tonnes of chalk.

A soot graffito records the last day of mining: April 17, 1912. There is a legend that one of the last of the miners, William Rowson, disliked the overseeing agent, and set light to his house, killing him, his wife and children, and all the mine records. This is substantiated by an illustration, scratched into the wall, of a burning house with a stick-limbed agent waving his twiggy arms in the flames. Alongside are the initials W.R.

The Herts and Middlesex Bat Group investigated the caves at the time of their discovery, and identified two rare species, Natterer’s and Daubenton’s. More recently (2011) the Group identified Natterer’s in St. Giles Church at the east end. The caves provide an ideal winter hibernation site, having high humidity and constant air movement. That accounts for another roosting site popular with bats, a Church belfry (St. Giles’ bell tower hosts Brown Long-eared Bats).

Mrs. Campbell passed the Editors a newspaper cutting from which this information is drawn. Unfortunately the newspaper and its date are not recorded, but I was told about the caves in 1988, which suggests that the article was published about then. My informant (now sadly passed on) indicated that the location of the caves was a closely guarded secret, to protect the bats in their winter quarters.
William Marsterson