This Turkish cave boasts some rather bizarre associations with the King of Mount Olympus.

This top secret Chinese military megaproject is the world’s largest human-made tunnel structure.

The attic of a four-century-old canal house hides a clandestine church.

Famed for its pie, this eatery has a hidden tunnel that allowed Catholics to secretly walk to church during the Reformation.

This amazing ancient cave was kept secret for seven years to protect it from the public.

Just your average Marie Antoinette–themed speakeasy in an abandoned 19th-century subway tunnel.

Approximately 300 million years ago, a massive coral reef existed in what is now the largest limestone cave in Japan.

The walls of these four caverns are filled with art that dates back to the earliest human presence in Europe.

This Cold War–era underground city includes offices for Mao Zedong and Lin Biao.

Two ancient Buddhist temples crown this dramatic rocky pinnacle.