Of all these intriguing curiosities, most interesting for adventurers is surely the lava tube — long, cavelike passages underground, arching in a near-perfect circle so ideal for traveling through, one might think they were man-made. These caves were formed millennia ago as the outside of a giant lava flow hardened, but a still-liquid center traveled through it like an underground river. Until the river stopped, leaving behind a circular tube unlike any other naturally-formed subterranean cave.
One such tube is the Coconino Lava River Cave in the Coconino National Forest, near Flagstaff, Arizona. The cave is massive – big enough for a person to walk through with dozens of feet overhead – and features the striking, telltale circumference of a lava tube. Spelunkers won't find stalactites or stalagmites decorating this hall of echoes – just the smooth arch formed by liquid-hot rock as it traveled through.
The unexpectedly smooth surface area can actually be hazardous, as the difference in temperature between the cool subterranean cave and the hot desert above ground is so great, condensation often forms and makes the walls and floors of the Lava River Cave very slick.
Coconino's Lava River Cave isn't the only lava river cave in the world, nor is it the largest even though it stretches for more than a mile through solid rock. But one extremely notable feature that continues to fascinate visitors and geologists alike is the unique "Y-intersection" deep within the tunnel, at which two massive tubes combine into one, creating a unique Y-shape out of three identical tubes. This leads to a dizzying mirror effect that can sometimes be disorienting to inexperienced explorers.
The cave is very close to the surface aboveground, with holes punctuating the ceiling at times, creating a surface treacherous for walking above the cave, but providing beautiful pillars of light from outside while traveling within it. This makes the Lava River Cave one of the most beautiful subterranean sites in the western United States.
