A former imperial sea-fortress and Soviet prison.

Mysterious craters may have been home to an ancient cult.

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

This disappearing geyser has been attributed to witchcraft, but really it's all about rainfall.

Historic slum of Tartu.

The gutted remains of this Estonian mansion are a monument to a tragic love.

In this historic region, homes and churches are carved into ancient volcanic rock formations.

The deepest underground city in Turkey was designed to protect 20,000 people behind massive stone doors.

Ruins of the Crusaders' headquarters.

Mysterious lost empire of the Silk Road.

A more chill but no less fascinating version of the famous ancient archeological site.

Charnel house at the foot of Mount Sinai.

The largest known Ancient Egyptian obelisk.

A salt lake dotted with steaming limestone chimneys.

A strangely forgotten claimant to the much contested title of "Source of the Nile."

Get tall with nature's lankiest beasts and their warthog friends at this Kenyan preserve.

Abandoned city hidden from the world by tropical forest and isolation on the coast of Kenya.

Human ancestor’s 3.6 million year-old footprints.

These naturally balancing rocks were pictured on every Zimbabwean banknote, up to the 100 trillion-dollar note.

The largest known meteorite on Earth was too big to move.

Ancient artists in the middle of nowhere.

This "dead marsh" sits among the largest sand dunes in the world, a forest frozen in time.

Held in a fort that once stood for colonization of the area, this museum now tells the history of its indigenous people.

Largest cave system in Africa.