Two very green residential towers cleaning the air in one of Europe's most polluted cities.

An ossuary built in 1210 to collect bones from the Ospedale Maggiore of Milan.

A spectacular Petrine Baroque summer palace commissioned for Catherine I by Peter the Great.

First point in the Struve Geodetic Arc, a 19th century undertaking to measure the Earth's size and shape.

The Guardian of Tallinn.

The secret cells in the basement of the KGB's Estonian "Grey House" have been preserved as a grim Cold War museum.

The oldest continuously operating pharmacy in Europe.

One of the largest busts in the world celebrates the German city's erstwhile namesake.

According to a local story, this tree's blossoms signal the end of war.

A Saxony castle houses one of the largest and most outstanding mineral collections in the world.

A museum honoring Germany’s cucumber mecca offers a great dill of gherkin history.

This jaw-dropping 19th-century bridge uses its reflection to form what appears to be a perfect circle.

These paved rills bring freshwater into Altstadt and could spell wedding bells for whoever stumbles into the waters.

Sitting in the basement of a castle, the largest wine barrel in the world is bigger than most apartments.

In this German city center, the streets have no names.

A former ironworks that was the first UNESCO industrial site.

A tiny chapel clings to a precipice above Germany’s gemstone capital.

The last bastion of Roman presence north of the Alps looms over a German town.

This medieval castle overlooking the Elzbach river looks just like it did hundreds of years ago.

This small museum nestled inside a souvenir shop is dedicated to Sweden's most well-known toy.

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

The world's largest model of our planetary system.

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

The library's graceful rotunda is like a bibliophile's temple.