This is the oldest Roman-made bridge in the city still existing in its original state.

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

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.

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

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

A Nesting Doll of Churches.

Jesus' footprints and the first catacombs in the world.

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

2300-year-old Roman road connecting the empire.

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

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

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

Rome's semi-secret peephole vista is also a former Crusader stronghold.

The yawning maw of this pagan visage is said to bite off the hands of liars.

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

Gaudí's famously flamboyant architectural style is on full display in this sprawling park and garden.

One of Antoni Gaudí's most classic buildings is well-known for its "dragon-back" design.

Construction of Barcelona's iconic (but controversial) church is expected to be completed in 2026—a century after the death of its architect.

The last remaining relic of San Francisco's glittering 1915 Panama Pacific Exposition.

The stones that traveled from Spain to eventually become a holy place for San Francisco's nature worshippers.

The oldest burial ground in the city, final resting place of city founders, Ohlone natives and murderers alike.

The remains of a ruined mansion memorialize the 1906 earthquake.

San Francisco's urban bison herd has persisted in the face of extinction, development, and disease.