A Lava Tube! Also, RARE Orientation
This here is the entrance to a lava tube out in the Cascades. These long tubes - caves, essentially - formed as flowing lava cooled on its surface and continued to flow underneath the cooled shell:
Lava usually leaves the point of eruption in channels. These channels tend to stay very hot as their surroundings cool. This means they slowly develop walls around them as the surrounding lava cools and/or as the channel melts its way deeper. These channels can get deep enough to crust over, forming an insulating tube that keeps the lava molten and serves as a conduit for the flowing lava. These types of lava tubes tend to be closer to the lava eruption point. (Wikipedia)
Click on the pic to see my set of photos from the cave, the Cascades, and RARE Orientation.
UPDATE: just added some photo descriptions for those pics. I'd forgotten to do that.