Upper and Lower Ape Cave Professional Review and Guide
"The longest known lava tube cave in North America, Ape Cave offers a challenging scramble with a surface return trail along its upper portion, while Lower Ape Cave is an easier out-and-back underground hike. Although other caves were known in the area as early as the 1890s, Ape Cave was not discovered until the mid-twentieth century.
The lava along the sides of the streambed as well as a crust over the top cooled enough to solidify, encasing the lava stream inside. Then the remaining liquid lava flowed on out of the tube, leaving the longest lava tube cave known in North America. At some time, possibly about 1480, a sandy lahar flowed into the cave’s main entrance. This lahar filled portions of Lower Ape Cave, blocking it about 4,000 feet below (south of ) the main entrance and leaving the sand and bits of light-colored pumice we see today."