Lava tubes are one of my favorite things. Formed as a result of cooling during an active flow, lave tunnels are a testament to the inherent lazy nature of the world. Even lava, if given a chance with take the easiest route, sometimes not even bothering to craft a new tunnel, instead flowing through a tunnel formed by a previous flow, using it as a means of easy travel and transport. When cooled, these caves become underground conduits that lead directly into the belly of the volcano. Cool.
Literally. Basalt is an amazingly efficient insulator and most caves stay close to freezing all year long, even in the desert. In eastern Oregon these caves were the first source of refrigeration for settlers eager to beat the heat. Early entrepreneurs made good money harvesting the ice from these caves and hauling it back to town in blocks for sale. Now, the caves are overrun with tourists, largely unprepared for the experience. Although it may seem to go without saying, these caves, like an others, are really dark, really cold, and offer questionable footing. Most people forget these things and find themselves shivering along in the dark with frozen toes. But it's worth it, even if it's just for the Lord of the Rings style ambiance.
There's a whole world down there...