The Catacombs of Paris, France, contain the remains of over six million people and are a popular spot for urban exploration.

The Poveglia Island in Venice, Italy, is considered one of the most haunted places in the world and is home to a decaying mental hospital.

The Chapel of Bones in Evora, Portugal, is decorated with human skulls and bones and serves as a reminder of the transience of life.

The Cabinet of Curiosities in Prague, Czech Republic, features bizarre artifacts collected by a 16th-century nobleman.

The Door to Hell in Turkmenistan is a natural gas field that has been burning since 1971 and attracts adventurous visitors.

The Cu Chi Tunnels in Vietnam were used by the Viet Cong during the Vietnam War and now offer a glimpse into the country's history.

The Chapel of Saint Gildas in Brittany, France, is only accessible during low tide and features a mysterious statue of a mermaid.

The Secret Annex in Amsterdam, Netherlands, is where Anne Frank and her family hid from the Nazis during World War II and is now a museum.

The Coober Pedy underground town in Australia was built by miners seeking refuge from the extreme heat of the Outback.

The Winchester Mystery House in California, USA, was built by a widow who believed that the spirits of those killed by her husband's rifles were haunting her.