BASE Jumpers Double the Dare Over the Edge: The newest addition to tandem aerial sports is BASE jumping off a cliff. MOAB, Utah - Castle Valley spread out about 1,500 feet below, and Mario Richard scooted his toes to the edge of the cliff. It can be an awkward shuffle when there is another person fastened to the front of him. Chris Hunter for The New York Times BASE jumping in tandem in Moab, Utah. Some have estimated that 10 to 15 BASE jumpers per year, on average, have died over the past decade. The height from Parriott Mesa flattened the rugged topography into a wrinkled blanket of swirled reds. Sandstone spires, like exclamation points of the gods, spiked the landscape. On the horizon, the La Sal Mountains provided a snow-tipped backdrop. "The product, what I'm giving people, is the sensation of leaving a cliff," Richard said. Richard, 46, asked his passenger for a thumb's up. Larry Harpe, a 42-year-old climbing guide who had spent many nights in a sleeping sling thousands of feet above the Yosemite Valley in California, had never been more terrified. For weeks, his thoughts were occasionally punctured by the dread of this moment. He slept fitfully the night before. Minutes before the jump, he was too nervous to sip from the whiskey flask he carried in his pack. "I've spent 20 years learning how not to fall," Harpe had said in the predawn dark, hours earlier. It's the last thing I want to do. Harpe's toes tickled the cliff's edge. He felt the weight of another man on his back. He and his churning stomach wanted to give a thumb's down. Harpe's thumb pointed up. "Three," Richard said. The countdown. No turning back. "Two," both men said. One. On "and" they crouched as a single unit. On "jump," they leapt. It would be over soon. Getting Attached BASE jumping is the extreme younger cousin of sky diving, which many probably think is extreme enough. Rather than dropping from planes, however, BASE jumpers fall from objects attached to Earth. That is the acronym: buildings, antennas, spans (bridges) and Earth itself. Instead of jumping from a plane thousands of feet in the air, BASE jumpers usually fall only hundreds of feet, giving them less time to sort out any in-air problems. Risk is exacerbated by the presence of the object itself - the launching point instantly becomes an obstacle - and the sometimes precarious landing zones, in places like river gorges. It is considered, even by those who do it, to be extremely dangerous. Yet danger is difficult to quantify in a sport with few regulations and no governing body. While credible BASE instructors usually require a new participant to have performed at least 100 sky-dives before overseeing a launch off a fixed object, there are no rules. Anyone can do it without oversight, and no one can reasonably estimate how many jumps are performed around the world. Statistics that are loosely tracked are the known fatalities. Some have estimated that 10 to 15 BASE jumpers per year, on average, have died over the past decade, for many reasons: pulling a chute too late, using improper gear or slamming into a cliff among the main ones. Recently, the biggest surge in deaths has been among jumpers wearing wing suits. The numbers nudge upward with increased participation. Now comes tandem BASE jumping, following the evolutionary arc of tandem sky diving 20 years earlier. Sky diving from planes was once the province of daredevils. But it found mass appeal after the advent of the tandem harness and other technological upgrades, dually credited in sky diving circles to Ted Strong and Bill Booth in the 1980s. Suddenly, with no experience or expertise needed, anyone could climb off a couch, take a 30-minute lesson and fall out of a plane while attached to an instructor. Millions have. The number of sky divers has mushroomed in the past 20 years, and an increasing percentage of the first-timers have been tandem jumpers. There are roughly half a million tandem jumps in the United States each year, according to the United States Parachute Association, and the numbers continue to grow.