This man-made mound monument was built out of soil from all across the Polish empire.

St. Mary's features one of the most spectacularly decorated cathedral interiors in all of Europe.

Poland's oldest surviving university building holds a wealth of historic science instruments including some used by Copernicus.

Beneath Kraków Old Town's main square lies a hologram-filled medieval market vampire graveyard wonderland.

Stockholm's decidedly unique public transit system has doubled as the world's longest art gallery for decades.

A king's bloodstained shirt is among the gruesome collection of artifacts displayed below Stockholm's Royal Palace.

The discovery of an old wall led to the creation of this fascinating museum about the city in the Middle Ages.

Sweden's oldest animated sign is still lighting up the skyline today.

It houses the remains of a 17th-century version of the Titanic.

This gem from the industrial age helps visitors and residents alike traverse hilly Stockholm.

Stockholm's deepest subway station is also a stunning ecological wonder.

A pair of connected skyscrapers form this symbolic gateway.

A misnamed well, with a deep dark past.

Destroyed Serbian army headquarters sits half crumbled in a largely revitalized Belgrade.

Nikola Tesla's ashes are held in a golden sphere, surrounded by his works.

The unusual sculptures are believed to be powerful political symbols.

A Serbian church lit by chandeliers made of weapons.

In the hinterlands of Serbia's capital, a giant reflective mushroom houses an aeronautical history of the former Yugoslav republic.

A former imperial sea-fortress and Soviet prison.

An empty relic of the Moscow Olympics crumbles away in Estonia's capital.

The Guardian of Tallinn.

An 18th-century Catholic church converted into an eccentric temple of music and bacchanalia.

Disused seaplane hangar gets a second life as the jaw-dropping home of Estonia's museum of maritime history.

A preserved 18th century homestead in Western Pennsylvania marks the spot where the Whiskey Rebellion began.