Subway Cave is a lava tube half a mile north of Old Station in the Lassen National Forest, and it is one of the easiest genuinely wild caves in California to walk into.
Developed stairs lead down into it, there is parking and there are picnic areas, and guided tours run seasonally at weekends. What there is not, at any point, is light. The self-guided trail runs about a third of a mile and the cave is completely dark, so a torch is not optional. The floor is rough and jagged, which means proper shoes rather than sandals.
It holds a steady 46 degrees Fahrenheit, which is the other thing people underestimate: a light jacket makes the difference between a pleasant walk and a cold one. No hard hat is needed and there is no crawling. Pets are not allowed.
There is no fee. Donations go into a locked tube at the trail near the parking area.
The season is set by snow rather than by a calendar. The cave usually opens around late April once the snow has melted, and the published season runs April to October, dawn to dusk.
It sits in the Hat Creek Valley, and the Forest Service groups it with the Old Station Visitor Information Center, the Spatter Cones Trail and the Hat Creek Rim Overlook, all within a few miles. Taken together they explain the volcanic country the tube runs through better than the cave does on its own.
Lassen National Forest covers about 1,700 square miles of northern California, and this is one of its most visited single features.
Other places on the map within a couple of hours.
