A formerly abandoned mental institute, now back in business.

An abandoned Gold Coast-era mansion once intended for the exiled ruler of Albania, who never arrived.

This extensive collection covers thousands of years of Polish military history.

Dubbed "Wistom" by locals, this abandoned factory is an urban explorer's dream.

This open-air museum was originally a World War II testing ground for German missiles and rockets.

A statue honors the children who fought Nazis in Poland.

A mysterious structure left behind on the Polish mountainside, sometimes called "Hitler's Stonehenge".

A tribute to the Polish insurgents who fought in the failed attempt to end their city's Nazi occupation.

An abandoned Nazi explosives plant founded by the inventor of dynamite is now an underground museum.

The site of the first battle of World War II.

This Nazi ruin was the site where Hitler was almost blown up by a suitcase bomb.

The former Nazi concentration camp stands as a museum to remember this dark chapter in European history.

A monument to the "father of modern warfare" Gustav II Adolf.

After a nearly successful attempt on Hitler's life, the conspirators met their fate in the courtyard of this Berlin building.

The largest brick bridge in the world.

The ruined remains of the airport Claus von Stauffenberg departed in his failed attempt to kill Adolf Hitler.

The prototype for all Nazi concentration camps.

Terrifying derelict military hospital once housed a recuperating Hitler.

The 1936 Summer Olympics site is a crumbling remnant of sportsmanship and a history its host country would like to forget.

Thousands of coffin-like pillars make up this controversial Holocaust memorial.

A state-of-the-art medical facility from the Imperial era is now a derelict ruin.

Once the world's largest reinforced concrete structure, this abandoned bridge now beckons urban explorers.

North America's oldest quarantine facility was built in response to Philadelphia's 1793 yellow fever outbreak.

This little-known Revolutionary War fort is supposedly haunted by the ghosts of its past.