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. Mount Lassen itself is the remnant of a much larger super-volcano, Mount Tehama, and was also the site of the largest volcanic eruption in North America during the twentieth century. The Subway Cave, however, is a completely safe location where the effects of volcanism can be viewed and explored. As a matter of fact, the Subway Cave is not actually a cave, but a lava tube, created during the area's volcanic past. In terms of geology, lava tubes like the Subway Cave are created during volcanic eruptions that cause large lava flows.
In such cases, the lava on top of the flow eventually cools and solidifies into rock. However, underneath this newly created rock, hot lava continues to flow, creating "tubes" and "tunnels", which eventually empty out into other locations. Years later, through various processes; usually erosion, the then empty lava tubes are discovered by either people or animals (or both). California is a great spot to visit lava tubes and lava caves due to the active volcanism that have shaped the state; and while the Subway Cave does not have the light features that the Lava Tube has in the Mojave National Park, it is much wider, and longer than its southern cousin.
Directions: The Subway Cave is located directly off of Highway 89, North of Lassen National Park. It is a quarter mile North of the junction of State Route 44; and it is well signed by the Forest Service on both sides of Highway 89. From the parking area, the trail is readily apparent, and is a 1/3 of a mile long. The trail heads through part of the National Forest, before descending a series of stairs into the Subway Cave. From this point, visitors can pass through the cave, and loop around back to the trailhead via the trail, or choose to walk through the cave; and then return back through the cave.
Tips: Even though it should be obvious, visitors to the cave should be prepared for total darkness and cool temperatures(it is a cave), with flashlights, headlamps, and jackets. It is also worth noting that the cave is also only open from April-November. This is also a great spot to stop at in my opinion, on the way to Burney Falls, the eighth natural wonder of the world.
More Information: The National Park Service has a great PDF about the cave that can be found HERE.