ORANGE CITY — Sixty-five feet below the surface, at the bottom of the crevasse formed by the headspring at Blue Spring State Park, lies a bed of manatee poop. Shelves of white limestone line the walls, and a strong current surges up through the narrow fissure.The water flow makes diving to the bottom only a little more difficult for Karen Mirlenbrink, who has spent an inordinate amount of her 38 years in the water. A Dunedin-based free-diving instructor, she gives lessons to everyone from surfers to people in the military. In between, she finds time to surf and paddleboard.In her classes, Mirlenbrink emphasizes the mental side of free diving. The most powerful swimmers can only go so deep without the right mind-set."The calmer you can be and the more you can focus on exactly what you're doing, the more you can understand what your body is going through," she said.This becomes important after a couple of minutes without breathing and under many feet of water, where death is never too far away.Laura Ottenberg is one of Mirlenbrink's three friends who accompanied her on the dive at Blue Spring, located on the St. John's River in Volusia County. She said practice — like in all other sports — determines how far you can go."You kind of learn your limits, and you can push a little harder every time," she said.A diver's breathe-up, or preparation strategy leading up to a dive, also determines the success of the dive, Ottenberg said.Free diving can be deadly, but there are ways to stay safe. Divers should always go with a buddy, Mirlenbrink said, because even the most experienced free divers can black out or lose consciousness from a lack of oxygen circulating to their brain."Your buddy is your lifeline," she said. "If something goes wrong, your buddy needs to be there to save you."Part of the allure of free diving is the quiet. Without a breathing apparatus, divers don't emit noise. Far below the surface, not much else does, either.Divers also need little equipment: mask, snorkel, set of fins and weights. Wet suits also come in handy at cold-water locations."Free diving is a completely different world than scuba diving or snorkeling," Mirlenbrink said. "You can hear all the animals, you see all the animals. They're not scared of you. They'll come right up to you."The sport has taught Mirlenbrink to slow herself down and maintain lower stress levels outside the water, too."I have better focus, because so much of what we do here is bringing yourself into the present moment and knowing what you're doing right now," she said. "If you're distracted, if there's something going on, you're never going to be able to dive well."At the Blue Spring dive spot, a tree hangs over the crevasse, partially obstructing its opening. Combined with the current and the cramped space, that makes the dive more advanced, Mirlenbrink said, though worth it for the thrill — and the manatee poop.