A once-glowing grotto at the bottom of Waikapalae Wet Cave.

A prehistoric landscape hidden in the cliffs of Maha'ulepu.

These ancient sacred grounds granted amnesty to those who managed to reach them.

This industrial area was the dumping ground for massive amounts of trash, some of which washes up as tiny bits of sea glass.

What remains of the summer palace of King Kamehameha III.

A beach made of semi-precious green sand on the Big Island of Hawai'i.

These picturesque Hawaiian falls are home to a rare species of fish that uses a special sucker to climb up cliffs.

Earth's tallest volcano is sacred to Native Hawaiians, and home to a dozen world-class observatories.

This black shoreline is made up of exploded lava particles.

This iconic 1958 tower welcomes air travelers to Tucson with neon-lit letters and architectural style.

A monument to the one-armed geologist who led the first documented expeditions through the Grand Canyon.

A geological mystery on Camelback Mountain.

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

This Arizona observatory is famous for investigations into Martian life, and as the place humans first spotted Pluto.

See massive fossils that are over 200 million years old in northeastern Arizona.

This 4,000-foot-diameter hole is touted as "the most well known, best preserved meteorite crater on Earth."

Ninety feet off the valley floor is an ancient dwelling put under protection of the U.S. government by Teddy Roosevelt.

Ancient pueblos built into the base of a natural canyon wall.

The iconic boulevard is the perfect place to catch sight of a bicycling skeleton, a levitating Satan, or a chef with his head stuck in a pizza.

One of Antoni Gaudí's most classic buildings is well-known for its "dragon-back" design.

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

Bold black and white motifs of life and death add a dramatic flair to this 17th-century crypt.

The Roman city was the birthplace of various emperors and is home to one of the empire's most impressive architectural feats.

The final resting place of the great navigator, or maybe his brother.