A marvel of Byzantine engineering under the streets of Istanbul.

An illuminated manuscript comprising the life's work of a lone monk, inexplicably decorated with a portrait of Satan himself.

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

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

Archduke Ferdinand II's collection of rare objects is considered one of the oldest museums in the world.

According to the local folklore, the myriad colors of this lake are the result of a lovelorn wizard's blunder.

Acrosticism and palindromes on this randomly placed stone.

A star-shaped fortress town fort built in the geometric model of the Renaissance utopian city.

This Tuscan lake hides a 13th century town that is still visible when the water recedes.

These caves are home to a small church and were used to age cheese.

Built in 1925, this glass-roofed shopping arcade is known for its modernist Italian architecture.

A makeshift shrine to the late rock star is still kept up on the nondescript façade outside 1 Logan Place.

An unusual sculpture with a meaning shrouded in mystery.

This abandoned Brutalist treehouse is a hidden architectural relic.

This enchanting 600-year-old tree is so magical it inspired the beloved tale of Pinocchio.

This caldera was once associated with the Etruscan god of the underworld.

This open-air mosaic gallery honoring an English artist took hundreds of volunteers more than seven years to create.

This bewitching Babylonian goddess haunts a hallway of the British Museum.

Eighty million natural history specimens call this gargantuan museum home.

According to legend, this tomb holds the secrets to time travel.

This beautiful deer park was built so King Charles I and his court could go hunting while escaping a deadly plague outbreak.

This ancient oak plays host to carvings of magical elves, gnomes, witches, and other creatures prominent in local folklore.

The fences outside some of South London's estates are made from recycled World War II stretchers.

William Blake claimed to have painted this gothic masterpiece after encountering this nightmarish being in a vision.