Lava tube or underground river?
Lava River Cave is a lava tube cave in northern Arizona’s Coconino National Forest. At approximately 0.75-mile (1.21 km) long, it is the longest cave of this kind known in Arizona.
Geologists believe the cave was formed sometime between 650,000 and 700,000 years ago when molten lava erupted from a volcanic vent in nearby Hart Prairie. The top, sides and bottom of the flow cooled and solidified, while lava in the middle flowed out, leaving a hollow space to form the cave.