One of the longest lava tubes in North America, the Ape Cave Lava Tube, is in southwest Washington near Mount St. Helens.
Ape Cave is in the Gifford Pinchot National Forest south of Mount St. Helens, the Washington volcano that erupted in memorable fashion in May 1980. The lava tube wasn't created by the 1980 blast, however; a logger discovered it in the early 1950s, and local Boy Scouts known as the Apes did the first extensive surveys of the tube before providing the cave its name. It's believed the tubes were formed about 2,000 years ago, but these lava formations are unusual for this part of the world – most volcanoes in the area don't produce the kind of fast-flowing lava that formed the Ape Cave tubes.
There are two lava tubes at Ape Cave. The upper tube is larger, although the lower tube is easier to explore. Ape Cave is the longest continuous lava tube in the country at about 2.5 miles in total, and one of the longest on the continent.
Be sure to carry a powerful flashlight when you explore the Ape Cave Lava Tubes, and wear a jacket – it's essentially always 42F in the cave. The floor of the tubes is uneven, and although the lower cave is considered “easier” to hike it still requires attention at all times.