Well, the answer used to be 'Shaft!', but it's not unconnected, as our Vince runs an embryonic blog called Thanet Underground, all about the smugglers' caves that are dotted around the Ile. It seems as if intrepid Vince likes nothing more than to spend his weekends investigating dirty great shafts and gaping holes.
There's one story on his site about a rumoured tunnel that stretches all the way from Pegwell Bay to Margate. Apparently a cave cove called Illingworth tried to navigate this tunnel back in 1938, but discovered it took hours just to struggle from Pegwell to the Lord of the Manor. Plus ça change!