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CAVES: Volunteer interpreter Frank Binney calls it the secret cave in the Point Reyes National Seashore, and it’s one of a few he explores during low-tide excursions.   Dave Bunnell

Waves crashing on the shores of the Point Reyes Peninsula are like a swinging door to a network of caves and beaches that few human visitors ever explore. These “secret caves and hidden beaches,” as park interpreter Frank Binney calls them in his Point Reyes National Seashore Association-sponsored field seminar, are accessible only during negative low tides less than a dozen days a year. And though Mr. Binney, a Woodacre resident, has been offering the…