Hidden beneath the streets of London is a secret subway that only carried mail.

An enormous, 30-story-deep hole in the middle of Guatemala City.

This floating cinema brings new meaning to the phrase "going out" to the movies.

A former French military respite turned jungle tourism hotspot.

On stormy days debris from this unfinished and abandoned skyscraper rains down on the Bangkok streets.

Virgil's many-mouthed gateway to the underworld.

A submerged archeological park still holds the wonders of a Roman Sodom.

The industrial monstrosity known as Bagger 258 is a colossal mining machine built in the DDR - but now left for dead in a German field.

A gallery of underwater art that can be visited by man and fish alike.

A complex, nine-layer maze on the grounds of a Venetian doge's playground.

The oldest of Venice's plague quarantine stations, designed to keep the medieval city safe from disease.

A small island less than half a mile from Venice is a forbidden island with a dark and twisted past.

Venice's "Island of the Mad."

Tiny castle in the middle of the Rhine made you pay to pass.

A German apartment building hosting as many trees as human occupants.

World's only telecommunication tower with a wooden pinnacle.

A rare water-powered funicular, it uses a huge tank of water and gravity to do its work.

This half-finished "city of toys" was almost Italy's own Las Vegas.

A late sculptor's last project was to turn this abandoned cement factory into a castle-themed amusement park.