The second-largest cave chamber in the world was discovered in 1898 by a 16-year-old and a friend known as "Pothead."

The perfectly preserved underground rooms where Churchill plotted the war against Germany.

This community on a small Venetian island has taken to painting their houses in bright neon hues.

The ancient tomb acts as a calendar that keeps perfect time.

This cemetery has graves dating back to the 17th century, including the city's oldest carved tombstone and an ominous cryptogram.

Giant, natural amphitheaters made of delicate geological formations called "hoodoos."

The gorgeous library at Trinity College is home to remarkable treasures of Irish identity.

An unusual round church in London with a Templar past.

Hundreds of lucky felines haunt the ruins where Caesar was murdered.

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

The largest hot spring in the United States is, as the name suggests, a stunning show of natural color.

The deepest lake in the United States, and once the site of epic destruction that lives on in legends.

The ancient dwellings of the Puebloans, located in the cliffs of majestic mesas.

A beautiful geyser under environmental threat.

The first declared National Monument in the United States.

Towering basalt columns, a creation of fire and ice.

Elevated freight railway turned wildly successful urban park.

In all of the 29 years it was in operation, no prisoners ever escaped successfully... or so they claim.

Famously misunderstood wonder of the ancient world

Macabre astrological automaton clock dating to the late 1400s.

This ornate Victorian marketplace was the setting for Diagon Alley and the Leaky Cauldron in the Harry Potter films.

Try not to get slimed in Tribeca.

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

A trash dump made beautiful by nature's power.