Take a walk through Nahuku, known as the Thurston Lava Tube, a 500-year-old lava cave located at an altitude of 3,900 feet on Kilauea Crater in Hawaii Volcanoes National Park. Lava caves like this are formed when a river of lava gradually builds solid walls and a ceiling. When the lava flow stops and the last of it passes downhill, a cave is formed. These caves can be a few feet high and only yards long, or they can stretch for miles with high ceilings. There are several lava tubes you can visit around the island but Nahuku is the most easily accessible and is a fantastic example of a massive lava cave.

Check out this video tour of the Carlsbad Caverns in New Mexico. Carlsbad Cavern includes a large limestone chamber, named simply the Big Room, which is almost 4,000 feet (1,220 m) long, 625 feet (191 m) wide, and 255 feet (78 m) high at its highest point. The Big Room is the fifth largest chamber in North America and the twenty-eighth largest in the world.

Bryce Canyon is not a single canyon, but a series of natural amphitheaters or bowls, carved into the edge of a high plateau. The most famous of these is the Bryce Amphitheater which is filled with irregularly eroded spires of rocks called hoodoos.

Dry Tortugas National Park is a national park about 68 miles (109 km) west of Key West in the Gulf of Mexico. The park preserves Fort Jefferson and the seven Dry Tortugas islands, the westernmost and most isolated of the Florida Keys. The archipelago's coral reefs are the least disturbed of the Florida Keys reefs.