Ape Cave on the flanks of Mt. St. Helens is a great example of a lava tube. It was formed over 2000 years ago. Surveyed at 13, 042 feet it is over 2 miles long.
The cave is open year round and the entrance fee to the park gets you in to it. Prepare for a wet, cold environment no matter what the outside air temperature is and bring strong flashlights with spare batteries, water and extra layers of clothes. Young children should be kept under observation at all times.
When we went to the cave we hiked to the upper entrance and came down flow as it was easier than the alternative lower entrance
Ape Cave Map