A marvel of Byzantine engineering under the streets of Istanbul.

In this historic region, homes and churches are carved into ancient volcanic rock formations.

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

A detailed pachyderm supports Rome's smallest Egyptian obelisk.

This ancient market is often referred to as the oldest shopping mall in the world.

More sculpture than building, this architecturally significant temple was built on the spot of Saint Peter's crucifixion.

A hospital where antique dolls are given new life.

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

The final resting place of the poets Shelley and Keats.

A master of trompe-l'oeil creates a stunning, tourist-fooling dome out of a flat ceiling.

The sacristy of this church contains remains and relics of members of the Aragonese royal family.

San Gennaro, the patron saint of Naples, greets pedestrians between these narrow city streets.

This medieval city gate is now a blend of the past and the modern world.

An unusual "skull with ears" sits in the crypt of this small church tucked away at the end of a narrow alley.

In a city that boasts four separate inclined cable railways, this is one of the busiest funicular lines in the world.

A popular legend claims that the famous pizza was invented at a restaurant in this alley.

Every day is like Christmas on this street where the traditional Neapolitan nativity scene comes to life.

These "anatomical machines" are only one of the many strange items in this Italian crypt.

This shining multicolored metro station deep under Naples is said to be the most beautiful in Europe.

An old department store turned Nazi prison turned artist commune.

For patriotic Russians, this memorial honoring the Soviet soldiers who died liberating Berlin in 1945 is the most sacred location outside of Russia.

This monumental work of art symbolizes the coming together of a previously divided city.

An imperial church bombed to ruins during WWII now stands as a jagged memorial.

A monument to the three wars that led to German unification towers within a Berlin traffic circle.