One of nature's most well-scheduled phenomenons resides in Wyoming's Yellowstone National Park.

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

A beautiful geyser under environmental threat.

The ghostly remains of what was once the world's richest copper mine.

The king of U.S. high points towers over every other peak on the continent.

Roadside collection of 65,000 signs.

This decommissioned DC-3 now forever points into the oncoming wind.

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

One of the best-preserved Roman amphitheaters is still used today, but for less brutal events.

A musical instrument played by the sea.

A chain of 16 wondrous lakes formed as a result of the gradual dissolution of soluble layers of bedrock.

A miniature train-ride through Slovenia's largest cave system.

Castle and secret tunnel withstood a siege until its owner was killed by the Holy Roman Empire, while sitting on the toilet.

This one-time health resort was used by aristocrats around the world.

This massive subterranean fissure is known as the "underground Grand Canyon."

Two stone columns flank the grand Venice square, but there were supposed to be three.

Thousand-year-old Viking runes adorn an ancient Greek lion at the entrance to the Venetian Arsenal.

A sinister pyramid in the Frari contains the heart of the famous neoclassical sculptor.

Go inside to really appreciate this 500-year-old technological masterpiece.

The ruins of the largest gladiatorial school in Rome are largely hidden under modern streets.

The meaning behind this triumphal Roman arch was a mystery until the early 20th century.

An 18th century fountain and two columns are all that remains of Rome's forgotten river harbor.

Its interior is adorned with some of the best examples of early Christian mosaics.

Ancient ruins hidden underneath a modern luxury hotel.