This massive library holds what was once the largest book collection in the world.

The shell of a medieval fortress offers commanding views of the area it once protected.

The heroic tale of a remarkable pup who refused to leave her master's side, no matter what.

These 12th-century catacombs carved into a mountainside were a place of mysticism and Christian hermitages.

Amsterdam's former hotspot for public executions and dissections.

A little marble compass above George Washington's (empty) tomb in the Capitol marks where D.C.'s four quadrants intersect.

"Congress may voluntarily remain in session throughout the summer, in order that our Congressmen may be protected from the intolerable discomforts and dangers of the ordinary outdoor weather!”

The discovery site of the "Capitalsaurus," the official dinosaur of Washington, D.C.

Corn-inthian columns with a uniquely American take on neoclassical architecture.

This massive stained-glass masterpiece is thought to be the largest Tiffany dome in existence.

This alley in downtown Chicago held the bodies of over 600 people after the fateful Iroquois Theatre fire.

These ornate arches show how the course of the River Thames has changed.

A 300-year-old tea shop that brought tea to the English people, not to mention the Queen herself.

Preserved portions of Athens's ancient defenses are on display in a subterranean parking garage.

The iconic Chicago sculpture hides a beautiful and easily overlooked plaque on the backside.

This memorial plaque remembers a strangely shallow maritime disaster known as "Chicago's Titanic."

A 400-foot-tall tall map of Chicago makes this once-ignored building stand out.

Once a year, a sandbar rises from the depths just in time for the world's weirdest match.

The oldest inn in England served brew to the crusaders.

A charming 17th-century alley is lined with secondhand bookstores and antiquarian shops.

One of the best collections of Byzantine art housed in a magnificent, 19th-century neoclassical home.

This airport also offers travelers the chance to view thousands of years of Turkish history.

A dead saint, a deadly poison, and a mysterious holy ooze.

The smaller of the three towers of San Marino is a former prison inaccessible from the ground level.