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.
In the far Northeast corner of the State of California is the best National Monument you've never heard of: Lava Beds National Monument. While the monument has many interesting historical, cultural, and geologic features to visit, it is primarily the home of the Medicine Lake volcano, which is the largest volcano by volume in the Cascade Range. For over 500,000 (1/2 million years), the Medicine Lake volcano has been erupting; and is a large shield volcano. While there is evidence of over thirty separate lava flows from the Medicine Lake volcano that can be viewed in Lava Beds, one of the most prominent lava flaws is the Devils Homestead. This lava flow originated from the Fleener Chimneys portion of the park around 2,000 to 8,000 years ago, and is considered an aa flow, as it is basaltic in composition and now has a blocky, uneven surface. Today, while the area has some growth in between the hardened lava, to me, it looks like the surface of the moon, and is one of the more surreal places to visit within the park.