Trevor Lawson takes a deep look at a South Mimms cavern

South Mimms, home of the legendary M25 service station and it's accompanying traffic jam, seems an unlikely venue for a caving expedition. Yet beneath a very ordinary looking farm nearby are a series of gigantic caverns which, until recently, had lain hidden since before the First World War. From the surface, you would never guess what lies below. Only three narrow, vertical shafts provide access to the vast underground chambers, although Caver John Huston, who rediscovered them almost by accident, believes that somewhere close by is a collapsed shaft large enough to accommodate horses and carts.

The reason is that the caverns are actually a vast chalk mine, a breathtaking monument to labourers who spent their entire lives wielding pick-axes by candlelight during the 1800's to supply the raw materials of calcium carbonate and flint to the local building trade.

Virtually nothing is known of the history of chalk mining in the Chilterns. Another huge tunnel system, discovered near Windsor last year, shows that it was a local industry of significant importance in the 19th century, yet it remains an enigma. Most local people around South Mimms have no idea of the mine's existence, although there are now more than 1,132ft of tunnel known, up to 40ft high and 32ft wide. It is one of the largest unsupported chalk mines anywhere in the world, representing a volume of chalk and flint exceeding 7,700 cubic metres.

And among the few who do know of the mine's existence and precise location, there are rumours that the excavations are much larger. A surface pond indicates subsidence in a second mine, and an entire field behind the farm is pitted with hollows, suggesting a gigantic system of tunnels below. Geologists might argue otherwise, claiming that chalk is too soft and unstable to support such excavations. But the miners were extremely cunning.

The tunnels were started close to the surface, and the diggers worked their way down into the floor, deeper and deeper, creating the shape of a huge church vault to give the structure stability. A century on, the vaults, emptied of some 23,000 tonnes of chalk, stand as firm as they did in the 1800's.

A soot graffito on the roof of the youngest tunnel records the last day of mining: April 17, 1912. "There are several carvings in the walls showing the mining agent, who wasn't particularly popular," explained John Huston, as we descended into the abyss. "Miners never liked their agents, but legend has it that in this case one of the last miners, William Rowson, set light to the agent's house, killing him, his wife and children and destroying all the records relating to the mine." Sure enough, scratched into a wall is an illustration of a burning house with a stick-limbed mining agent grimly waving his twiggy arms in the flames. Alongside are the initials W.R.

By a quirk, this act of arson has actually been beneficial to certain members of the local community: bats. A search through the records in County Hall reveals that only a well is listed for the area, probably for tax-evasion purposes. A mine was a very taxable asset, but wells came free. Consequently, the mine has remained undiscovered by authorities seeking a suitable site for a nuclear fall-out shelter, and cavers and bat enthusiasts alike intend to keep it that way.

The Herts. and Middlesex bat group has just completed £6,600 worth of work on the mines entrances, clearing the third shaft of debris to improve air circulation in the tunnels. Two nationally rare species, Natterer's and Daubenton's Bat, use the tunnel system as winter quarters. The numbers are surveyed monthly by bat group members, who descend 80ft to the pit face to reach the crevices in which the bats roost.

Contrary to popular belief, a stable atmosphere does not make an ideal hibernation site for bats. A dynamic environment is essential, with high humidity and constant air movement. By clearing the third shaft, Patty Briggs of the bat group hopes to increase the number of bats using the site. "This is the first mine of its kind in the Home Counties to be capped and renovated in the way," she explained.

"The tunnels are so large that two shafts on their own cannot maintain enough air circulation to suit bats. With a third shaft clear, we hope to have 100 roosting here in the coldest months." It is ironic that a mine with such a morbid and mysterious history should experience so lively a renaissance. For the bat, it is equivalent to the Brave New World below ground envisaged by H. G. Wells's artillery man in "War of the Worlds". While development and intensive agriculture increase the threats to bats' long-term survival above ground, they at least have a secure retreat in the bowels of man's bygone excavations.

Thanks to Richard Vidler for providing this for me !