Unlike caves carved by the slow passage of water over rock, the Ape Cave is a lava tube created by an eruption of Mount St. Helens about 1,900 years ago. During a volcanic eruption, channels of smooth-flowing, pahoehoe lava may crust over to form lava tubes. When the eruption ends or the lava is diverted elsewhere, the molten rock drains and leaves partially empty conduits beneath the ground; lava can also erode downward, leaving empty space above the flow.
The largest contiguous lava tube in the Western Hemisphere, Ape Cave's two-and-a-half mile (12,810 feet) tunnel wasn't explored until 1946, when a local Boy Scout troop - the St. Helens Apes, for whom the cave is named - discovered it. During the cave's creation, hot gases trapped in the tube remelted the walls and ceiling to form lava stalactites, or "lava-sickles." One cooled ball of lava floated down the tube's flow until it got stuck between ledges; the lava drained and left it perched ten feet overhead. During a later eruption, mudflow deposited sandy, volcanic debris in the tunnel; water dripping into the cave has swept away sediment and left behind "sand castles."