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 excursion for the last seven years, it always holds some surprises. “One of the cool things is that people in the class see things I can’t see or have questions I can’t answer,” he said. “I have to come back and research so that by the next class I’m ready.” Mr. Binney is an avid caver who’s been ducking into crevices since he was a young boy in Missouri. As an adult, he helped conduct surveys at Mammoth Cave National Park and Carlsbad Caverns National Park. He has volunteered with the seashore since the early 1990s. Last Friday afternoon, Mr. Binney led a retired National Park Service lawyer, a school teacher from San Geronimo, a former park ranger and Dave Bunnell, a photographer of caves and mines. The sky was clear enough to see the Farallon Islands; red-tailed hawks, tule elk and barnacles added to the scenery of the nine-mile hike. The journey’s crown jewel was what Mr. Binney calls “the” secret cave: a collapsed sink hole formed as the result of a sea cave cutting through the rock beneath it. It’s a natural wonder the size of a small high school gym. Looking upward from inside, the sky is a small circle of blue. And, though the cave and the nearby beaches are seldom visited by humans, they are strewn with garbage: buoys, bottles and every sport ball imaginable. After about an hour of exploration, the impending closure of the crashing doorway kept the group on schedule. The next “Secret Caves and Hidden Beaches” field seminar takes place on Wednesday, March 8 at 11 a.m. The non-membership fee is $60. For information, visit ptreyes.org.