Chiselhurst caves !! Bank holiday weekend explorations on Saturday ! Much recommended place to visit, great guided tour too :) Quoting from wiki : Today the caves are a tourist attraction and although they are called caves, they are entirely man-made and were dug and used as chalk and flint mines. The earliest mention of the mines is around 1250 and they are believed to have been last worked in the 1830s.
During the early 1900s, the caves became a popular tourist attraction. In World War I they were used as an ammunition depot associated with the Royal Arsenal at Woolwich. They were used for mushroom cultivation in the 1930s.
During World War II, when the aerial bombardment of London began in September 1940, the caves were used as an air raid shelter. Soon they became an underground city of some 15,000 inhabitants with electric lighting, a chapel and a hospital. Shortly after VE Day the shelter was officially closed. One baby, christened Rose Cavena Wakeman, was born in the caves.