8. Mammoth Cave National Park, Kentucky
It's the longest cave in the world. No other known cave comes close. About 360 miles have been surveyed so far, and geologists estimate that the cave system’s total length is about 1,000 miles.
The cave runs through 350-million-year-old limestone, composed partly of shells deposited when Kentucky was at the bottom of a shallow sea. A wide river later replaced the sea and left a layer of sandy sediment on top of the limestone. Water dissolves limestone more readily than sandstone, so over millions of years rivers and rainwater have seeped through and eroded the limestone, creating caves. You can see all the classic cave features here: stalactites, stalagmites, crystals of gypsum, blind fish, narrow passages and “bottomless pits,” which park rangers point out to scare children.
7. San Andreas Fault at the Carrizo Plain, California
For a fault that regularly topples buildings, rips apart bridges and kills people, the San Andreas can be surprisingly hard to see. The best place to observe the 800-mile-long fault is along the Carrizo Plain, west of Los Angeles. The land is undeveloped, dry and fairly barren, so the trenches formed by past earthquakes haven’t been worn away by erosion and plants don’t obscure the view.
The San Andreas is the grinding, lurching plane of contact between the Pacific Plate and the North American Plate. The Pacific Plate is pushing south-southeast and the North American is pushing north-northwest, rubbing uncomfortably against each other as they travel in opposite directions.
6. La Brea Tar Pits, California
In downtown Los Angeles, just off Wilshire Boulevard, is an unprepossessing geologic feature: a pit of oozing oil. The sticky asphalt has been trapping animals—including the occasional hapless pigeon—and preserving their skeletons for at least 40,000 years.
The museum at the tar pits displays wall after wall of dire wolves, saber-toothed cats, Columbian mammoths, ground sloths and camels. The skeletons are plentiful and beautifully preserved (the animals sank pretty quickly in their death throes). It's the best place to get a sense of the animals that roamed North America before humans arrived.
5. Mount St. Helens National Volcanic Monument, Washington