I had heard about the Ape Caves, formed from lava about 2,000 years ago, from a friend a while back. Just this week two friends and I got together and hiked the length of the upper cave. It was so cool. We bought our pass, got there, and went to find the entrance.
We were a little confused and ended up wandering/backtracking to figure out where to go. Eventually we went in the main cave, down some stairs, and behind the staircase. We came to a large room and the entrance we wanted was on the far right of the far wall. It took a bit of wandering to find this.
There are quite a few things to maneuver around, and there were two parts where it took someone strong to give us a hand up. I'd recommend going with someone strong, otherwise, I'm not sure how you'd do those parts. There wasn't much to speak of for handholds or footholds there.
And I'd recommend bringing a headlamp as one of your sources of light. I liked not having to worry about the flashlight attached to my wrist and just have my headlamp for a few parts.
The cave is completely dark except for the entrance, one skylight, and the exit, so we didn't take too many pictures.
And finally, a view of Mt. St. Helens from the trail above the caves.
I'd recommend trying it sometime. It was fantastic. You can find information about it online.