The genteel Zone 5 suburb of Chislehurst hold a secret beneath its feet. Thirty metres below ground is a garagantuan 22-mile complex of man-made caves that were first carved as flint and chalk mines in the late medieval period (and there are those who theorise they’ve much older, dating back to Ancient and Roman Britain).
In more recent times the caves have been turned to all sorts of purposes, including an ammunition dump in World War I and a mushroom farm; during World War II, they acted as Britain’s largest bomb shelter; post-war they’ve serves as a popular filming location, notably used by the a ’70s Doctor Who Mutants and the ’00s TV series Merlin were filmed here.
Most relevantly, it is by far the biggest cave complex in the capital and you can do a tour here, which is very cool and fun and while you should check the opening hours, it’s fine to just rock up and join a group. The 45-minute lamplit tour covers just a mile of the tunnels, but it’s plenty to get your teeth into; children will enjoy locating the Druid Altar, the Caves Church and the Haunted Pool. Just make sure they don’t wander off.