Two stone columns flank the grand Venice square, but there were supposed to be three.

One of Catholicism's four major basilicas has a series of papal portraits that are said to predict the end of the world.

The secret medieval communication system will whisk your words above the noise of the crowd.

Stone memorial that's said to predict the death of the pope and the site of the Cadaver Synod.

Fountain where visitors can earn a license to be mad.

An open-air museum in a mountain valley features works designed to blend in with—and disintegrate back into—their natural surroundings.

Gods clash with machines in a power plant regenerated as a museum.

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

Italian poet Dante Alighieri is entombed in this small mausoleum in Ravenna, and his hometown of Florence has never been too happy about it.

This palatial spiral staircase was so unusual the name "snail" was given to the noble family who built it.

This bronze boar's snout has been rubbed to a golden sheen by visitors seeking good fortune.

This hill made entirely of ancient Roman jars was simply a garbage dump to the people of the time.

Romans built the world's tallest artificial waterfall in 271 BC.

Bell tower juts out from a village buried under a lake.

This private skyway is filled with art and was expanded explicitly so that Hitler could appreciate a river.

This ritzy Italian market is built around a statue of an unforgiven heretic who was burned for his belief in an endless universe.

A Nesting Doll of Churches.

The remains of a town so thoroughly destroyed by French forces that the citizens simply left.

Virgil's many-mouthed gateway to the underworld.

Natural cathedral designed by Italian artist Giuliano Mauri.

Filled with goldfish swimming in a shallow pool of sea water, this ancient crypt is more wishing well than burial site.

Over 50 exquisite hand-painted Renaissance maps decorate a Medici storage room.

Jesus' footprints and the first catacombs in the world.

The engineering wonder and pleasure dome of a much-maligned emperor still holds undiscovered secrets.