These caves secretly held national treasures during World War II.

Gladiators once walked this ancient, subterranean passage.

A Byzantine-era church hidden in a wooded area below the Acropolis.

This gem is often wrongly overshadowed by the larger, newer Metropolis church in Athens.

Where commuters walk through remnants of an ancient Greek necropolis.

This small 16th-century church in Athens is almost entirely surrounded by a modern building.

Legend says this sacred tree was a gift from goddess Athena.

Heralded as the world's first theater, these ruins are often overlooked as they reside in the shadows of the Acropolis.

The railway carts travelers to central Athens's highest peak, which offers stunning vistas of the city.

This ancient Greek place of assembly and marketplace is being revealed layer by layer below the modern Athens cityscape.

Monasteries on rock pillars, once accessible only by frayed ropes.

An artificial date palm stands as a memorial to Warsaw's Jewish community.

A tribute to the Polish insurgents who fought in the failed attempt to end their city's Nazi occupation.

This hidden collection of everyday objects illustrates life within communist Poland.

Krakow's own version of the Hollywood Walk of Fame.

A statue commemorating a Polish dog who took his role as man's best friend very seriously.

Inside this Krakow church, an explosion of psychedelic color.

St. Mary's features one of the most spectacularly decorated cathedral interiors in all of Europe.

This Polish cave has housed a dragon, a brothel, and an enduring legend.

Real bones which are said to have belonged to a Polish dragon.

Twelve grave-faced marble angels guard the emperor's tomb at Les Invalides.

The vast, legendary catacombs hold secrets much stranger than stacked bones.