SURF'S up - really up. Jumping from an airplane at 14,000 feet, some Australian skydivers have mastered the technique of "sky surfing," or riding a surfboard in the air until their parachute pops open.The proof is in the commercial. The skydivers are featured in a new international Coca-Cola advertisement that began showing in Australia just this month but has been airing in the United States for a few weeks longer. The Aussie ad shows surfers in an airplane over the parched Australian outback. One thirsty surfer looks out the window and sees an outback roof with the familiar red-and-white Coke sign painted on it. Before you can say "Bonzai Pipeline," he's out the door with his surfboard, his mate close behind. Looks easy, but it's not. One of the surfers, Steve Whalan, says it took a few hundred jumps to master the technique. The difficulty is in controlling the board as it drops (surfer attached by foot straps) at a rate of 1,000 feet every six seconds. "The tendency was for the board to want to flip you upside down," says Mr. Whalan, a senior instructor at the Sydney Skydiving Center. The Aussies put out an SOS to other jumpers around the world, asking for suggestions. The French proposed using large webbed gloves for better control and the Americans thought of giving the board a V-shaped keel. Neither would work for the commercial: The producers wanted as authentic a surfing "look" as possible. So Whalan and seven other jumpers worked on their technique by starting with small, skateboard-size boards and working up to the real kahuna. It took a week of jumping to get everything just right for the ad. Whalan and his fellow jumpers were at it seven or eight times a day. (The people in the plane in the commercial are actors - they didn't jump.) It cost just under $800,000 to make the ad. What's it feel like? Whalan describes it as a gliding feeling. "It felt like standing on a column of air, not a wave," says Whalan who is not a surfer. Sometimes it was bumpy, sometimes smooth. "It was exhilarating, but very tiring," he adds. It was also pretty cold - no wet suits were allowed, and the hair gel used to keep everyone's hair in place made it feel like they were encased in ice in the 14-degree F. temperatures. Since the commercial has run, Kathy Walsh, a spokeswoman for Coke, says the soft-drink company has received dozens of phone calls, including some from people who wanted to try the Big Surf in the Sky. Whalan says it is not likely to catch on, however. "It's too hard," he says, but adds: "I may be wrong."