I hiked the lower Ape Cave which is .75 miles to the end and .75 miles back to the steps. It's a very cool experience. Definitely come prepared with a good headlamp and good footwear and the warmer clothing (it's a steady 42 degrees in the cave). It's pitch black once inside and there are uneven surfaces. Occasional water drips from the ceiling. A really unique experience to do in beautiful Washington State. The cave is really tall in sections and it's amazing to see what time and lava have carved out.
To go in the summer, you need a $2 reservation made in advance on recreation.gov (there's no cell signal out there). You also need a parking pass which you can purchase there or use a NW Forest or America the Beautiful pass.

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