Cu Chi Tunnels, Vietnam Basilica Cistern, Istanbul, Turkey

In the late 1940s communist forces began digging a network of tunnels under the jungle terrain of South Vietnam during their war of independence from French colonial authority.

The tourist attraction we know today was also once the stronghold of the Viet Cong who fought against the USA and the South Vietnamese government in the 1960s and 70s.

At its peak, the tunnel system stretched for more than 155 miles underground from the outskirts of Saigon all the way to the Cambodian border. It included countless trapdoors, living areas, storage facilities, weapon factories, hospitals, command centres and kitchens.

Some of the narrow tunnels have since been (slightly) widened, so visitors can wriggle through them on their hands and knees. vietnam.travel

This colossal tank, the largest of many underground water reservoirs in Istanbul, was built beneath the ancient basilica by Byzantine Emperor Justinian I in 532 AD.

But the grand, cavernous cistern lay undiscovered until the 16th century, when a visiting Frenchman was intrigued by locals who could fish from their basements. The cistern was cleaned and renovated in 1985 and opened to the public two years later. Visitors explore the site on raised platforms, while carp still swim in the waters. With a serene but spooky atmosphere, it’s one of the city’s most popular attractions and featured in the 007 film From Russia With Love. yerebatan.com