Exploring in the Nitnat area we were told of some hidden caves to check out. The access is off a logging road, but you could drive down it every day and never know anything was down there. There is a small bridge where I've dropped the pin, which just looks like a small ditch crossing. Look a bit closer however, and you'll see that there is a 100+ft vertical drop, only about as wide as your arm span. This is how you know you're in the right spot!
Park your truck just before the bridge on the right hand side, and look for a very small ingrown path leading down a steep ravine. It's quite short, but very steep so I recommend good footwear (that you don't mind getting wet). At the bottom you'll find the river. Walk along the river heading upstream, and keep a sharp eye out for a crack in the rock on your left. We almost walked past it the first time as it's quite hidden. The crack opens up into some large caves. Don't let my camera flash fool you, it's *pitch black* in there. Bring a flashlight! As you go a bit deeper into the cave, you'll hear water flowing - follow the sound and you'll emerge into the main canyon.
You can get through the caves and peek into the canyon on foot, but to really get into the full dome farther in you need to do some swimming. When we did this it was 30+ deg outside, but it was still ice cold water. You can skirt around some of the water but there are places where you just have to do a ~20-30 ft polar bear swim. It's worth it. The canyon itself is super cool, with towering walls that go up at least 100 feet.
The pictures we took didn't really do it justice, but it's a very cool spot!