The most unique thing about Lava Beds National Monument are its many lava tube caves, which were created by lava flows over a period of fifty thousand years from 10,000 to 60,000 years ago. As the lava flowed from the Medicine Lake volcano, the surface cooled and solidified. Underneath the surface, lava continued to flow to various areas, eventually emptying the “tube” underneath. Over the course of time, the rock cooled, cracked, and collapsed, producing openings to the surface. Today, there are over 700 lava tube caves in the National Monument, of which over twenty (20) can be explored.
On the Northern border of Lassen National Park in the Lassen National Forest is the Subway Cave, a remnant of Northern California's volcanic past. This portion of California is part of the Cascade Range of mountains, ancient volcanoes that shaped the geology of the region thousands of years ago, and continue to shape the region even today.