At the foot of a waterfall lies a peaceful swimming lagoon and evidence of over 60,000 years of human habitation.

Barcelona's historic 19th-century sewers, whose foundations were laid in medieval times, are open to the public for exploring.

Hundreds of stone stairs and a winding medieval bridge connect this haunting Spanish island to the mainland.

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

A series of caves, tunnels, and wells filled with fascinating figures and symbols.

Nine caves filled with recently-discovered archaeological finds under the houses and the streets of the most ancient district of an old town.

A network of underground water channels that dates back to the Middle Ages.

A mountain cave is the site of archaeological findings and Paleolithic rock art.

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

This Roman underground canal was the longest tunnel ever built until the late 19th-century.

These artificial caves crafted by ancient Romans were elaborate fish farms for moray eels.

Italy's third-largest Roman arena gives a peek into the underground operations beneath the ancient spectacles.

A 2,000-year-old engineering wonder hides beneath an Italian town.

The pristine waters crossing this forest form a hidden waterfall among the dense vegetation.

A forgotten "city of water" underneath the famous Trevi district.

Ride a boat through this magnificent underground world, which now doubles as a theater.

An underground paleo-Christian burial site composed of the remains of several cemeteries and Basilicas.

This picturesque Umbrian city has a subterranean side that has long remained hidden.

A sunken idol welcomes parishioners to Davy Jones' Locker.

Dug as a secret royal escape route, the tunnel became a wartime bomb shelter and dumping ground for vintage cars.

This Italian foot path winds through the mountains via 52 individual tunnels.

A complex, nine-layer maze on the grounds of a Venetian doge's playground.

The location of the last witch trials of Italy, known as "The Salem of Europe."

A labyrinth of ancient tunnels hidden below the city holds the ruins of 2,500 years of history.