Join us as we explore Ape Cave National Monument, a chilly, pitch black lava tube over 2 miles long! There’s a lower cave which is rated as easy, and an upper cave which travels over large rock piles and requires scaling of an 8 foot lava fall! Ape Cave formed about 2,000 years ago, when an 8-mile-long lava flow known as the Cave Basalt poured down the southern flank of Mount St. Helens . Check out the longest lava tube in North America!