Gibraltar is just 6.8 square kilometres in area, but the size of its role in WWII is immeasurable. Beneath its distinctive landscape lies no less than 55 kilometres of hidden tunnels and caves, a subterranean city carved out by a military workforce. Also, under London, 21st-century engineering innovation continues the legacy of Victorian pioneers paving the way for the ultimate super sewer and Europe's largest and most breath-taking natural cave systems - The Postojna Cave in Slovenia.

A look at some of Switzerland 300,000 subterranean bunkers built across history. The extraordinary creation of a local entrepreneur that's shrouded in mystery in Liverpool, England. And, a network of tunnels created by man and today used by adventurous subterranean thrill-seekers deep beneath Slovenia's beautiful landscape.