Actually, there's many active caving clubs in the area! In Washington, there's the Cascade Grotto, in Oregon, the Oregon Grotto , the Willamette Valley Grotto , and the Oregon High Desert Grotto all make trips to this and other areas - Check out the states' map at: National Speleological Society
These guys will hook you up with the right equipment and show you some of the nicer, hard-to-find caves out there! All (most) of the caves out there have been mapped, which helps find hidden rooms and entrances, etc...
Ivan, after Dynamited your're gonna be disappointed - that's the "best" lava tube out there. "Real" caves are in limestone, not lava, and around here you'll have to head to Vancouver Island, Cave Ridge (in WA), or the Marble Mountains (in CA) to find something better. That said, there's hundreds of small lava tube caves on St. Helens and Adams - find a flow and follow it - you may even find something virgin! (map it and send it to the grotto!)