The Edinburgh Vaults in Scotland
Located under Edinburgh's now infamous South Bridge, which began construction in 1785 and completed three years later, these vaults were originally built for tradesmen to serve as workspaces. Due to the tight budget for the construction, it was never water proofed and later deteriorated from leaks and dampness. As time went on, business moved out, and the poorest of the growing city inhabited these spaces, albeit barely surviving Though the structure itself was an impressive feat, the quality of workmanship slowly deteriorated over time, and consequently, so did the quality of life for its inhabitants. Before too long, the vaults became a hub for violent crime, prostitution, gambling, and disease. They were eventually closed to the public in the 19th century up until 1988, when they were rediscovered because of excavation efforts. Nowadays, visitors are able to be led on guided tours through the area, where guests have reported feeling cold gusts of wind, hearing footsteps, having rocks thrown at them, and feeling a presence holding their hands.

Catacombs of Paris in France
Due to severe overcrowding in Paris' cemeteries, and rising public health concerns over an abundance of decomposing bodies in the area, King Louis XV prohibited burials of any kind within the city's limits in 1763. As a solution, the 65.6-foot-deep Catacombs, which had existed as tunnels since the 13th-century, were officially blessed in 1786 and continued to accept bodies until 1859. It now houses the remains of 6 million bodies within its walls and is open for public tours year round. Last year, lodging website Airbnb hosted a contest in which the winner had the opportunity to sleep over in the spooky underground site on Halloween,much to Parisians' dismay, who felt this dishonored those buried in the ossuary and could potentially damage the heritage site.

The Cincinnati Abandoned Subway in Ohio
Conceived as an effort to alleviate traffic above ground and prove that the city was capable of completing a project of this magnitude, the Cincinnati subway system—a project formally called the Rapid Transit Loop—began its construction in 1920. But due to soaring costs coupled with the city's financial losses that came with Prohibition, and bad infrastructural planning, building stopped in 1928. The tunnels have been left abandoned since (save for a brief effort to revisit the metro project in 2002) and have become home to a lot of the city's homeless. Though the space is not haunted, it is still unsettling to walk through, and sits as a sore reminder of missed opportunities.

The Basilica Cistern in Istanbul, Turkey
Built in the 6th century, the cistern (referred to as 'Yerebatan Saray' in Turkish during Ottoman times, or 'The Sunken Palace') was constructed to serve as a water reservoir for Byzantine emperor Justinian I's palace and the city above. At 459 feet deep and 229.6 feet wide, this massive underground structure is supported by vaulted ceilings and 336 Corinthian- and Dorian-style marble columns—two of which have the mysterious, massive heads of Greek mythological figure Medusa carved at their bases from the Roman period. Though neglected during the Ottoman era, the underground gem was renovated in 1985 by the Istanbul Metropolitan Municipality. Today it is open to visitors as a museum, and maintains its enigmatic ambiance through dim lighting, traditional music, and some carp that now inhabit the pools.