Lake Shasta Caverns, a limestone cave system near Shasta Lake in northern California, is at least 200 million years old and was formed by flowing water that later drained away, leaving chambers filled with stalactites, stalagmites, soda straws, columns, and flowstone. First recorded explored by James A. Richardson in 1878, the caverns remained accessible only to a few spelunkers for decades through a tiny natural opening. Public access developed in the mid-20th century after Grace M. Tucker secured ownership and a tunnel project unexpectedly opened the Discovery Room. Now a show cave reached by lake crossing and mountain ascent, it became a National Natural Landmark in 2012.