A marvel of Byzantine engineering under the streets of Istanbul.

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

A sharp sculpture made of thousands of pieces of jagged glass.

Replicas of Apollo's leonine stone guard cats still line a sacred row in Greece.

The ancient latrines of the Greeks, in all their public glory.

A charming beach town in Turkey captures all the colors of the rainbow.

This crumbling medieval basilica once attracted pilgrims by the thousands to collect a miraculous dust that formed above the saint's tomb.

Seven early Christians were sealed into a cave where legend has it they slept unharmed for centuries.

Carvings in Turkey's Marble Road may have directed ancient Romans to a hidden brothel.

The alleged house of the Virgin Mary is tucked away in a park and nature preserve on Mt. Koressos in Turkey and is holy to both Catholics and Muslims alike.

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

Historic fortifications form the boundary of Vatican City, but it’s not entirely surrounded by walls.

Hidden beneath the famous St. Peter's Basilica is a veritable city of the dead that may hold the remains of St. Peter himself.

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

This stately building in Alexandria, Virginia was built by Freemasons to honor one of their most famous members.

It was once the base for a series of domestic terror attacks.

Legend has it that President Grant’s frequent drinking in the lobby gave rise to the term “lobbyist.”

A monument in Washington D.C. marks the spot from which all other roads were supposed to stem.

America's post-nuclear-attack chambers of Congress.

Visit a replica of the estate where Aaron Burr allegedly plotted a treasonous scheme against the United States.

The largest burial mound in the United States holds the remains of prehistoric West Virginians.

The crash of this great airship signaled the beginning of the end for dirigibles.

The building stands within a quiet, postcard-perfect New Mexico village.

The marker offers a rare chance to be in four U.S. states at once, though there's some controversy over its geographic accuracy.