The "Greatest Sewer" of ancient Rome is one of the oldest sewer systems in the world, and is still in use.

The underground sanctuary of a mysterious ancient cult.

A Nesting Doll of Churches.

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

Architectural monsters are devouring this palace's every door and window.

The crypts of Capuchin friars decorated with the bones of over 4,000 friars, including an entire "crypt of pelvises."

Roman-era ships unearthed...

Ruins of a 10th century church tucked away deep in a botanical garden.

This hidden Art Nouveau courtyard is an opulent feast for the eyes.

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

Art and armour fill every inch of this rich kid's Italian villa.

Caves full of Buddhist statues, incredible rock formations and a deserted lagoon.

Granny Smiths and Fujis can both be traced back to Kazakhstan, where apples still grow wild.

Villagers have recycled bomber fuel tanks from the Vietnam War and turned them into very, very strong boats.

These tombs were built into the side of a mountain so that ancient angels could reach the dead.

A cluster of granite granaries perched on stilts.

150 or so scrounged-up lamps, chandeliers, and lanterns light up this alley in central Athens.

A 19th-century shrine covered with prosthetic body parts as votive offerings.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

A nearly 500-year-old sculpture depicts a man eating a sack of babies, and no one is sure why.

Sacked by Vikings, blessed by William the Conqueror, dealt a fatal blow during the French Revolution—these ruins have seen it all.

In the town of Åtvidaberg, the midday sun sets off a loud cannon boom.

An ancient city exhumed from the depths of the ocean.

A submerged village appears when dam levels drop.