The largest free-standing aquarium in the world resides in a hotel lobby in Berlin.

Ruined 11th century abbey built by King David I in Edinburgh, Scotland.

One of only two commuter aerial tramways in the United States.

One of Antoni Gaudí's most classic buildings is well-known for its "dragon-back" design.

This eccentrically decorated palace is replete with grottoes, fountains, tunnels, and caves.

Whisper secrets in a busy station only to be heard on the other side of this acoustic arch.

Construction of Barcelona's iconic (but controversial) church is expected to be completed in 2026—a century after the death of its architect.

Read a book next to a Midtown waterfall.

The most visited, and possibly most controversial, museum in Germany.

The most iconic structure in Scotland has been home to bishops, colonels, and Sean Connery characters.

Macabre astrological automaton clock dating to the late 1400s.

This charming cobbled backstreet is the most Instagramable street in Paris, much to the residents' annoyance.

17th century streets hidden under Edinburgh were once a breeding ground for the black death.

Started as a kitchen garden, now the second largest botanical garden in the world.

Two stunning 17th-century libraries and a cabinet of curiosities.

This 1,300 year old monastery built atop a single rock was once only accessible depending on the whims of the tide.

This massive library holds what was once the largest book collection in the world.

Abandoned railway line circling the city of Paris.

Elevated freight railway turned wildly successful urban park.

Notorious artist squat renovated into legal studios.

The only structure left with direct ties to the Salem witchcraft trials of 1692.

France's most famous cemetery, with some of its most curious tombs.

A church of bones, decorated with 40,000 human skeletons.

These fantastically named natural pools look like something out of a Scottish myth.