Lava tubes are a common sight on the north flank of the Medicine Lake volcano south of Tulelake, California.  These caves formerly contained hot molten magma that drained out and left out this hollow tube which eventually cooled and solidified.  During the process, parts of the ceiling of the caves collapsed creating skylights or breakdown collapse sinks.  This section here is at two breakdown sinks, and b/w the two a portion of the roof remain intact creating this spectacularly balanced natural bridge spanning across, only a few feet in width and standing about 20-30 feet above me.  Behind looking back into the more intact portion of the lava cave one can see light from a ceiling skylight as well as another breakdown trench about 300 feet further in.  These types of natural bridges are unique & are abundant at the Natural Bridges pulloff in Lava Beds National Monument.