This is easily the dumbest category on this list because, obviously, if we tell you about it, IT WON'T BE A SECRET. So maybe you know about this one, maybe you don't, but it's not long, it's very easy, and the really secret hikes are where we grow our ganja, so leave us alone. It's part of Aliso and Wood Canyons Wilderness Park, which features 30 miles' worth of trails. The Dripping Cave portion is less than a mile long but forks off Aliso Canyon trail and Wood Canyon trail, so you're looking at about 5 miles round-trip. It leads to—surprise!—Dripping Cave, also known as Robbers Cave, where legend has it thieves used to stash their plunder. Now they stow it in the Hall of Administration in Santa Ana.