Bay seniors looking for an outdoor jaunt with very few fellow hikers might want to try the Vasco Caves Regional Preserve on the shoulders of Mt. Diablo. The rolling hills contain bizarre rock formations and vernal pools. Because of the fragility of the caves and their remnants of Indian rock art (dating back 10,000 years) visitors are only allowed on guided tours. The Preserve is also home to endangered red-legged frogs, tiger salamanders and fairy shrimp.