Gibraltar is just 6.8 square kilometres in area, but the size of its role in WWII is immeasurable. Beneath its distinctive landscape lie 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.
