The U.K.'s first underground railway line is now a lively thoroughfare for runners and cyclists.

An unusual slot canyon in the Aztec/Navajo Sandstone has a cross-section like an inverted old-fashioned keyhole.

These free-roaming horses were once destined to disappear from the land.

This 40-room dwelling tucked within a cliff was built by the basin's prehistoric residents.

This remote desert watering hole has a connection to a curious chapter of American history.

Nearly 300,000 acres of wildlife, natural splendor, and a hidden ghost town have been designated a national monument.

This isolated outpost was once the only place to cross the Colorado River for hundreds of miles.

An off-the-beaten path cave with an underground lake.

Rusting dinosaurs frolic at the entrance to this roadside "snake sanctuary."

Hidden in a Scotland museum is a copse of extinct tree stumps.

This huge concrete map of Scotland was built as a thank you gift from a Polish soldier.

These ancient sea caves hold Britain's highest concentration of Pictish rock carvings.

Described by William Wordsworth as a "World of Wonder."

An ancient stone wall formed by lava that was a frequent stop during funeral marches.

Bronze plates placed around this Scottish fishing village form a true-scale model of the solar system.

On this stunning Scottish island, you can walk in the footsteps of dinosaurs.

A glacial pothole was once a hideout for a notorious Scottish outlaw.

A strenuous, unofficial trail leads to spectacular views in one of America's most remote national monuments.

One of the most extensive and well-preserved collections of dinosaur footprints in North America.

These elusive, isolated rodents evolved into a distinct, locally iconic subspecies.

The remnants of a creative early method of wildfire prevention.

Although now a decaying ruin, this stone cabin's history includes several notable figures.

This ancient trail has claimed the lives of over 2,000 people.

This oasis-like spring in the Sonoran Desert is home to unique, endangered species.