Searching for a network of German military tunnels, hidden beneath Zhongshan Park in Qingdao, China.

North Korea’s Pyongyang Metro is the world’s most secretive subway. This report visits 5 metro s

The ANZAC Drain in Melbourne, Australia, is home to ‘The Chamber’ - meeting place of the Cave Cl

Beneath the streets of Beijing, China, lies an underground city designed to house 6,000,000 people.

The Leordeni 10 Fort is one of the 18 abandoned Fortifications of Bucharest, Romania.

Forgotten German military tunnels under Mount Fu just outside Qingdao, China.

A Cold War bunker beneath the Park-Monument of the Bulgarian-Soviet Friendship in Varna, Bulgaria.

A doomed descent into the cave known as St Vincent’s Well – Bristol, UK.

Beneath Kiev, Ukraine, there lies a complex network of Soviet storm drains, tunnels & mines.

The Monument to the Bulgarian-Soviet Friendship, sometimes simply referred to as the Russian Monumen