Further to Henry Dalton's letter (NCE 23 May) there is certainly a large complex of caverns and tunnels in the area of Blackheath Hill and The Point, and there is little reliable historical record of exactly where they are.
The tunnels are not only in the chalk - a few hundred yards to the north east there is a partially collapsed network of tunnels in the Thanet Sand that overlies the chalk. In the early 1900s they were described as 'a perfect maze of tunnels (running) for a considerable distance'.
Now only a relatively short network totalling about 100m of tunnel, generally up to 2m high and 1m wide, remains accessible. These tunnels were used during the war as unofficial air raids shelters, presumably by people who assessed the risk of collapse as being less than that from the bombs. I am not sure that I would have taken the same view.
Certainly any engineer undertaking construction works in the area needs first to carry out the proper ground investigations.
G Mathieson (F), George Mathieson Associates, 83 Blackheath Park, London SE3 0EU