Stockholm's narrowest alley is barely over two feet wide.

An ancient Viking runestone hidden in plain view in Stockholm's old town.

It is said that each of the white stones in the red facade of this medieval house represents the head of a murdered Swedish noble.

A spitting ode to a German serial killer.

A small but immensely extravagant palace built by the "Night King."

Walk inside a half mile long crevice right outside the town of Garmisch Partenkirchen.

The fairy tale castle of the "fairy-tale king."

This medieval castle overlooking the Elzbach river looks just like it did hundreds of years ago.

Swarms of dainty flying insects swarm into the Petaloudes Valley to get busy.

This secret coastal village is studded with pastel-colored houses all piled on top of one another.

Legend has it that this bronze orb, which sat atop an ancient Egyptian obelisk, once held the ashes of Julius Caesar.

An ancient Roman temple first dedicated to an emperor's wife is now a church.

This small, ancient Roman temple was once a church dedicated to the patron saint of penitents.

This ancient trail climbs the mountain where a Latin League cult worshipped Jupiter, its main deity.

The ruins of the largest gladiatorial school in Rome are largely hidden under modern streets.

This curious, round temple is the oldest still-standing marble building in Rome.

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

A treasure trove of Renaissance-era funerary art in a Roman church built on top of Nero's grave.

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

This large, circular tomb is the final resting place of the first emperor of Rome.

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

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

2300-year-old Roman road connecting the empire.

The only "Egyptian" pyramid in Europe and the legendary tomb of Remus.