Tranmere contains one of the largest and most expensive WWII air raid shelters in the country. It consists of a series of tunnels stretching to a total length of 6500ft and was designed to house up to 6000 people (many of them workers at the strategically important Cammell Laird shipyard) - however by the time they were completed, they were no longer needed as the threat of invasion has diminished. Costing an estimated £129,000 it contained bunks and seats for all these people as well as a canteen, medical wing, toilets and even space for a library, as Liverpoll Echo reports.
The tunnels were later used by the Ministry of food for storage, and were considered as a nuclear fallout shelter during the cold war era. The tunnels were finally sealed off in 1989 amid growing H&S concerns. The tunnels still exist however and building work in 2008 uncovered a shaft - allowing temporary exploration of them, before being sealed off again.
More pics at: Hidden Wirral Myths & Legends