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.

Yesterday was only the second time I had visited Ape Cave. 20 years ago, I did the lower cave, and yesterday I returned with 3 teenagers and one 9-year-old.
The lower cave was so easy and interesting that we decided to do the upper cave. The upper cave is harder than "moderate" but doable. Some spots were pretty scary for my 9-year-old, but the teenagers helped guide him through some difficult places. We were a pretty slow group, so we let some groups go ahead of us and some places. I got claustrophobic at some of the tight spots but made it through knowing the groups ahead of us must have made it through. I'm glad we did it but probably wouldn't do the upper again.
The one complaint I have is someone in the upper cave took at least 2 explosive craps in the cave right near the most accessible pathways. At first we smelled it then we saw it twice! It was pretty fresh and disgusting. I know you can't dig a hole in a lava tube or pack it out, but whoever did this is disgusting and ruined a great memory for me and my kids. 

Upper ape cave experience.
PSA: NOT for people who havent climbed in a while, have bad knees, or are carring a ton of extra weight or youll struggle to climb the 3 rock walls and verticle rope that makes you lift your own weight up 12 feet on a rope ladder 1.5 miles in. might be 2 miles in and out, but it's constant climbing, and upon finishing, you still have to walk back 2 miles to the parking lot. So, know your boundaries and limits!

Climate and climbing: Wear layers as it's 44 degrees year round. Also, 2 light sources, NOT your phone. we brought headlamps and flashlights. if you're looking to do the upper caves, wear a small or no backpack as you'll scramble up 3 very high rock walls, and lots of rocker that will question if you're on the right train (you are). No huge thru hiking backpacks, or you'll get them stuck on the rocks. also bring some gloves to climb up some rocks. This is a totally doable hike for experienced hikers and climbers, which is why it's labeled moderate.

Pointers: Two times, you'll question if you got lost (cave gets really small). You haven't. Keep going. On tye climb out and up the ladder you get to the top. take a LEFT and follow the trail of BLUE arrows back.

The upper caves are a blast and quiet as many don't brave this way, but it's worth it for people who enjoy climbing and hiking.

Trail of Two Forests: Do not skip this side jaunt. It's a peaceful boardwalk trail that shows the old tree wells from a forest long destroyed by a lava flow, all surrounded by the new forest growing on top of the lava.  

Ape Caves,  Lower: The lower cave is a very fun walk through a giant, and easily navigated lava tube. 

Ape Caves, Upper: My legs will bear the bruises of that adventure for weeks to come. There are countless boulder fields to navigate, many requiring 4 points of contact while crossing over deep gaps in the boulder fields. Some sections get very narrow and tight. Many people were hitting their heads on the ceiling while climbing through. The rope climb could use a new rope. The upper cave is a challenge, but it's worth it to make it through. It's simply amazing.  

Trail is in good condition. Return trail from the upper tube has some oddly broken trees that snapped up high and into the trail. Both have been maneuvered out of the path and removed. It is possible playful bear activity.