The Chislehurst Caves are actually man-made chalk and flint mines (not caves!) dating back over 800 years, with separate sections supposedly dug by the Saxons, Druids and Romans.
how it's used in Merlin
In the episode The Mark of Nimueh, the Caves stand in for the area under the castle of Camelot where the water supply is managed. The Caves might have also been used for the caves in the Forest of Balor that Arthur and Nimueh walk through in The Poisoned Chalice. However, I suspect that the scenes of Nimueh plotting at her font or well are filmed against a green screen in the studio, and the chasm where Arthur finds the Mortaeus flower likewise (perhaps using the set providing a ledge in the cave of the Great Dragon).
the place itself
The Caves are located under the south-eastern suburbs of Greater London. They were working mines until the 1830s. They have since been used as a tourist attraction, an ammunition depot, a mushroom farm, an air raid shelter, and a music venue.
links and details
The address is: Chislehurst Caves, Old Hill, Chislehurst, Kent BR7 5NB, England.
rating
I haven't yet visited the Caves, so I figured I can't really give it a rating yet. I might come back to this later!