Seen enough trees?  If you want a change of pace while visiting Sequoia National Park, go underground on the Crystal Cave tour.  Crystal Cave was discovered in 1918 by two park employees while they were out fishing on a day off.  It was opened to the public in 1940 and can only be visited on an organized tour.  It is a limesone cave which has evolved into marble in places due to pressure and temperature.  The result is beautiful pools reflecting exquisite colors and various crystal-like structures hanging from the ceiling and coming up from the ground.  Some of the more impressive sections of the cave are the "organ room" , "the curtains" and the "capitol building" so called for their sculpture like resemblances to their namesakes.  The basic tour lasts 45 minutes, is suitable for all ages and types and is available from mid June to early September.  Some longer tours offered only in the peak summer months are the "Discovery Tour" which is an expanded version of the basic and the "Wild Cave Tour" which lasts 4-6 hours, has special physical requirements and must be reserved ahead of time by mail as group size and dates available are very limited.  Tickets for the basic tour are NOT sold at the cave's entrance and must be purchased at the Lodgepole Visitor's Center at least 90 minutes prior to the tour you wish to take.  This is because it is about a 30 minute drive from Lodgepole to the cave area and then a 1/2 mile walk to the cave's entrance.  Tickets are nonrefundable so give yourself plenty of time to get there.  Also, be sure to bring a jacket or sweater as Crystal Cave has a constant temperature of 48 degrees.