One of the largest caves in the world open to the public.

This massive subterranean fissure is known as the "underground Grand Canyon."

Renaissance villa and gardens complete with a tormented colossus.

During World War II, this rice mill became a Nazi concentration camp.

A wunderkammer of gold and silver reliquaries and ancient saintly bones.

The complex has been abandoned since the Croatian War of Independence, barring a stint as a set for 'Game of Thrones.'

An ancient knight and a symbol of the city's freedom.

A full figure 4th century incorruptible saint lies in a glass coffin in Croatia.

This "flat geyser" bubbles up out of the ground periodically like a disappearing river.

This unreal Portuguese palace looks as though it's made from a pile of different castles.

A unique seaside cave where Aleister Crowley faked his own death.

A former steam-powered mill from the 1800s is now an industrial park with a giant steel dragon slide.

The ruins of an Estonian prison are drowning in the quarry lake where the convicts were once forced to work.

Ozymandias’ time-keeping temple.

Tubing through a cave lit with glowworms is a magical underground experience.