The Underground tours!!! So cool and spooky. Not sure about this particular company, but in general totally worth doing. Underground opium dens, prostitution, speakeasies and shanghaiing (see : http://en.wikipedia.org/wiki/Shanghaiing).
Dover Castle, Kent - one of the best castles I have visited and so much history.....used through WWII for many different purposes......there are 4 miles of underground tunnels that were used as a hospital during WWII.....built on the White Cliffs of Dover
Meaning 'Battleship Island' in English, Gunkanjima's real name is Hashima and it is one of 505 uninhabited islands in the Nagasaki Prefecture, about 15 kilometres from Nagasaki itself. It earned its nickname due to its resemblance to a military warship. Despite being off-limits to travellers, the island has become an irresistible magnet for urban explorers who go to extraordinary lengths to investigate and photograph the island's abandoned buildings.