A little-known, yet charming glacial lagoon.

The world's longest known cave system.

Watch the newest land on the planet form before your eyes.

When ecosystems collide in the arid Southwest.

These remote Florida islands have a history of sea turtles and sunken treasures, and one of the world's largest coastal brick fortresses.

The highest dunes in California are the most likely place in the area to hear the Singing Sands.

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

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

A gothic palace of endless stalagmites and pseudoscorpions waits within one of the world's most beautiful caves.

These millipedes glow not out of attraction, but as a not-too-subtle warning.

These ruins of a pre-Columbian cliff village are among the most beautiful and well preserved in New Mexico.

The illuminated falls are a gem in North Cascades National Park.

Sunset-colored dunes of volcanic ash make taking a poor photo impossible.

Massive mounds of golden sand look like a desert landscape randomly plopped in an Arctic forest.

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

A surreal landscape where fanged rocks rise from the ground and the ocean feels like a great mystery.

A throwback train ride transports passengers through one of the lesser-known national parks in the country.

For two weeks every June, fireflies in the Smoky Mountains perform an astonishing light show.

This Yellowstone geyser may not shoot as high as the nearby Old Faithful, but it erupts for considerably longer.

The hike through a narrow river flanked by the 2,000-foot-tall walls of Zion Canyon is one of the best in the world.

Quite simply the largest tree in the world (by volume).

The translucent turquoise hue of these natural swimming pools comes from a high concentration of minerals.

The same water that fills a luxury resort’s pools feeds these free hot springs in the rainforest.

A unique collection of more than 250 rocks and minerals located on a university campus.