Bay seniors who think they’ve explored every inch of every trail around here might be interested in being less superficial and try going underground. You can join up with the SF Bay Chapter of the National Speieological (a Scrabble 200-pointer) Society in Menlo Park and try your hand at spelunking, better known as caving. Membership is only $12. They offer extensive training before you disappear into a cavern. September 1 through 4th they will be trekking down to Winding Staircase in the Mojave Desert.