On Road Trip 2009, CNET News reporter Daniel Terdiman visited Craters of the Moon National Monument and Preserve near Arco, Idaho.

Formed from thousands of years of activity along the "Great Rift" of the Snake River Plain, lava "welled up," according to a National Park Service brochure, "to produce this vast ocean of rock. The most recent eruption occurred a mere 2,000 years ago, and geologists believe that future events are likely."

This smooth flowing lava is known as pahoehoe.

Click here for the entire Road Trip 2009 package