Lava Tube is a conduit where lava once flowed, cooled and created a cave-like hole in the rock above Lower Twin Lake. This geologic formation is commonly referred to as "Hole in the Wall." The Lava Tube is a popular destination for backcountry skiing: skiers begin from from the back side of Mammoth Mountain, an out-of-bounds area, and descend the experts-only slope through the rocky tunnel. Winter visitors are advised to view Lava Tube from the eastern shore of the lake because of avalanche danger. In the summer, you can walk around the lake for a closer look.