A marvel of Byzantine engineering under the streets of Istanbul.

The marble facade hides a little-known medical museum with early surgical instruments and rare copies of canonical texts.

Hanging within this medieval church are the bones of a beast allegedly slain by a fourth-century saint.

Artistic tombs, monuments, and a beautiful "Hall of Fame" lurk inside this cemetery.

The train platform where Jews were sent to concentration camps is now preserved as a Holocaust memorial.

"The prettiest love letters in the world" ... or so said Lord Byron.

The odd depiction of the Virgin Mary and baby Jesus is rooted in the Catholic Church's eradication of Catharism.

Small on the outside, big on the inside, this church is an astonishing act of visual deception.

Just across from the "Last Supper" painting is the famous painter's vineyard, a gift received for creating the masterpiece.

Long-abandoned art nouveau baths beneath Milan's Piazza Oberdan.

An ossuary built in 1210 to collect bones from the Ospedale Maggiore of Milan.

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

An ancient Greek monument and a Byzantine church coexist inside this cave.

View the ancient remains of buildings and the ancient "River of Hades" before hopping on a train.

A tiny Greek Orthodox church from the Ottoman era hides on a busy city street.

This small chapel holds the mummified heart of one of the heroes of the Greek Revolution.

Gladiators once walked this ancient, subterranean passage.

The only surviving choragic monument is a trophy awarded in 334 BC for the winning dramatic chorus.

A Byzantine church containing the holy remnants of three saints sits atop the site of an ancient temple.

After a catastrophic forest fire, a Greek artist transformed scorched tree trunks outside an old sanatorium into this haunting sculpture garden.

A preserved portion of an ancient Athens barrier is displayed in a hotel basement.

This graffitied 19th-century building was a prototype of European industrial architecture.

A sprawling collection of books and manuscripts about ancient Greece that would have put their famed libraries to shame.

The oldest standing Byzantine church in Athens is tucked within a monastery in the middle of an upscale shopping district.