Lava Beds National Monument

See, the monument lands are covered with over 300 lava tubes (caves) that were formed when various volcanos and cinder cones were active which can present some challenging fun to the first (second, third...) time cave explorer.
For us, the caves are great because it's an opportunity to crawl around on our bellies, get really dirty, explore all kinds of little passages, and have a lot of fun. Without tour guides, cave exploration takes on an entirely new meaning!
Source = An unofficial (but good) WWW page... Lava Beds National Monument page
CVO Menu - America's Volcanic Past - Lava Beds National Monument
Lava Beds National Monument (National Park Service)
CVO Website - Medicine Lake and Lava Beds Vicinity, California