1. The Door to Hell, Turkmenistan:
The "door" is a huge, gas-filled crater that's been burning nonstop since it was lit by Soviet scientists in 1971.
2. The Catacombs of Paris:
The remains of over six million people are found in Paris' dark, labyrinthine underground tunnels.
3. The cenotes in Mexico:
Natural sinkholes that were literally used for sacrifices by the ancient Mayans.
4. The Tunnel of Love, Ukraine:
Found deep in the forests of Ukraine, the Tunnel of Love was formerly used to transport industrial goods from a nearby factory.
5. The Richat Structure, Mauritania:
It was once thought to be the result of an asteroid impact, but it's now argued to be a deeply eroded geologic dome. The Richat Structure is also known as the Eye of the Sahara and is visible from space.
6. Mount Buzludzha, Bulgaria:
The former home of the Bulgarian Communist Party is now an abandoned saucer-like structure atop a mountain.
7. The Initiation Wells, Sintra, Portugal:
Located in the Quinta da Regaleira palace, The Initiation Wells are two underground towers that were once used for ceremonies, including Tarot rites.
8. Aokigahara Forest, Japan:
Also known as the Suicide Forest or Sea of Trees. Mount Fuji towers over this eerie forest.
9. Cappadocia, Turkey:
Vast networks of underground chambers and tunnels, some of which bear Byzantine frescoes. Incredible.
10. Masaya Volcano, Nicaragua:
This volcanic crater, which has a history of human sacrifice, was referred to as the "Mouth of Hell" by the conquistadors.
11. The tree roots of Meghalaya, India:
A normal wooden bridge would quickly rot in Meghalaya's subtropical climate — but these bridges consist of ancient tree roots and vines, and they're very much strong and alive.
12. Kolmanskop, Namibia:
A former diamond mining town in the Namib Desert where geological forces have buried many houses in sand.