A piece from KURG's page on defensive works.

"World War I
This marked a change in the pattern of warfare since the fighting took place in the trenches of France and Belgium, with no requirement for new fortifications in Britain. It did bring in a new development, however, whereby men of the Royal Engineers began to tunnel under the enemy trenches to lay explosive charges, hence the derivation of the word 'mine'. The base depot of the Royal Engineers was at Chatham and the area was used to experiment with new techniques of mining, since the local sand and chalk was almost identical to conditions at the Front. Very few of these trial mines were recorded at the time and, although it is likely that many collapsed subsequently, some still turn up in surprising places. The latest example was discovered following a collapse in a Gillingham back garden in 1988."

The also have a page offering help.

Here's links to an exploration in Gravesend which looks a bit similar - the word soakaway pops up - no-one's mentioned that yet.

Pictures of a hole

Or follow links to recent projects, Gravesend Hole. Most pages operate out of the same address.

Penny