Footage filmed near the western Polish city of Miedzyrzecz on Thursday reveals a Nazi fortress made up of long, complex tunnels know as the Regenwurmlager, translated literally as 'Camp Earthworm.' The underground labyrinth includes secret rooms and long tunnels stretching for over 100km (62 miles) at a depth of 20 to 50 metres (65-164ft). Three-metre (9.8ft) thick walls of reinforced concrete... Еще and steel cover what is considered the longest underground defensive complex in the world. The Regenwurmlager was built in World War II and it was mainly used for training troops, storage and as a rest area. It was abandoned in 1945 as German troops were forced to retreat from the eastern front.