(CNN) Why would a perfectly sane person parachute off a cliff?
That's what people asked BASE jumper Carl Boenish all the time.
Boenish -- a kind of merry prankster of BASE jumping -- devised some carefully worded answers to that question.
He had a romantic answer: "I'm glorifying mankind's beautiful spirit of seeking adventure," he'd say.
Or when he felt rebellious, Boenish would say: "There are many man-made laws -- that aren't laws at all -- that need to be broken. One is the belief that it's impossible to jump off a cliff."
And then sometimes Boenish would simply describe the feeling: "You have a feeling of freedom and power and confidence -- almost euphoria. You think, 'Wow, I feel like Superman.'"
Many people already know about BASE jumping through countless videos of these daredevils posted on social media.
But few realize that Boenish co-invented the sport in the late '70s and early '80s. He even helped come up with the name.
BASE is an acronym for buildings, antennas, spans (i.e. bridges) and earth (i.e. cliffs). Jumping from these places is considered more dangerous than skydiving from airplanes because there's less time to deploy the parachute or to recover if something goes wrong.
Now, almost four decades later, it's impossible to know exactly how many BASE jumpers there are. But a veteran of the sport told CNN affiliate KCRA he estimates that worldwide there are about 1,000 people who jump weekly and another 1,500 who do it once or twice a year. About 10,000 people have BASE jumped at some point in their lives, he said.
Boenish -- whose name rhymes with Danish -- was a fearless, friendly Californian with a big, snaggle-toothed smile who jumped so many times that he created a career and an entirely new sport.
Early in his life, Boenish worked as an electrical engineer for Hughes Aircraft, which was owned by aviation legend Howard Hughes. His boundless, energetic personality left him bored and restless with his chosen career, so he directed his energy and his engineering expertise into his two passions: filmmaking and skydiving.
After 1,500 parachute jumps from planes over 15 years, Boenish needed a new adventure -- and that's when he began parachuting off high cliffs.
In 1978, Boenish led a group of skydivers who jumped off a nearly 3,000-foot-high outcropping near the summit of the famed El Capitan, the near-vertical rock cliff at California's Yosemite National Park.
He wanted to capture the historic jump on film in a spectacular way: "I wanted to figure out a way if possible to film a person running off a cliff, but from a vantage point looking back toward the cliff," Boenish explained in CNN Films' documentary, "Sunshine Superman."
To do that, he built a 20-foot, angled aluminum ladder that he and some rock climbers attached to El Capitan's rock face. Boenish perched himself on a seat on the end of the ladder -- suspended over the ledge.
"He would go out there and then he'd have to get up the nerve and then swing 180 (degrees) so he could sit on that seat," jumper Kent Lane recalled. "And then you would hear his laugh and you were guaranteed that he was nuts.
"And he talked us into jumping off this thing, so we're all nuts."
Why the risk? For Boenish, filmmaking was an integral part of skydiving and BASE jumping.
"If it weren't for the camera, at least I personally probably wouldn't be that interested in skydiving," Boenish said. "It's really a sense of creativity that we do a lot of things in skydiving, just for the sake of the camera -- and hence, millions of people -- to see what we do."
After seeing what Boenish and his team accomplished, other parachutists jumped off El Capitan in the months afterward, leading to arrests. The park began requiring special sanctioning and permits. But the system broke down and authorities stopped granting permits in 1980.
Boenish taught his wife, Jean Boenish, to jump. And as they both learned, there were many other tall things around the world to leap from.
Imagine standing atop a skyscraper, teetering on the balls of your feet on the roof's edge and looking down to the street below.
"You're very apprehensive and you're nervous," said Boenish in the film. "But you know in your mind you can do it, even though your physical body says you better not do it."
Extreme sports psychologists say the fear of falling is essential to human survival, but oddly some of us are actually drawn to it.
Some of us actually want to jump.
"We're frightened, yet we're also excited at the same time. ... We release dopamine which provides us with a pleasurable feeling," Rhonda Cohen, a professor at Britain's Middlesex University and an expert in the psychology of extreme sports, told CNN last May.
"The first second you start accelerating incredibly fast, it leaves you almost breathless," Boenish said in the film.
In fact, every second we fall, our speed increases. (The rate of acceleration would be 32 feet per second per second in a vacuum. In air, it's not quite that fast, but it's still scary.) Each floor of the skyscraper whizzes by more quickly than the next. A jumper who falls for five seconds is traveling about 110 mph.
The body also experiences accelerating heart beat and rising blood pressure.
Finally, when the parachute opens and the jumper lands safely, something strange happens, Cohen said. It sound crazy, but the stress, fear and euphoria make us want to do it again.
Along with the thrills come the deadly risks.
"Base jumping is probably the deadliest sport in the world," wrote ex-BASE jumper Chris McNamara. "It is also probably the coolest."
McNamara wrote on his website that he left the sport in 2009, fearing he'd eventually die in a BASE jumping accident. He came to believe that "BASE jumping does not get safer with more experience."
No official statistics are available, but according to a fatality list maintained by BASE jumper website Blinc Magazine, 275 people have died BASE jumping since 1981.
That includes Carl Boenish, who lost his life during a jump in 1984.
The death rate is increasing. Half of these deaths have taken place since 2009 -- 27 in 2015 alone.
Last year BASE superstar Dean Potter lost his life at Taft Point, a Yosemite viewpoint not far from El Capitan. He and jumper Graham Hunt were reportedly attempting the leap wearing wingsuits.
Wingsuits -- which let jumpers glide through the air horizontally at fantastic speeds -- have lured many mountain climbers to try BASE jumping, as they did McNamara.
Developed over the past few decades, wingsuits have taken BASE jumping to an entirely new level compared to the days of Boenish. Because the suits are relatively new, McNamara and other jumpers say this is the "Golden Age of wingsuit BASE" -- a once-in-a-lifetime chance for jumpers with wingsuits to make their marks, or die trying.
BASE jumping has even gone professional, in a sense, with a team sponsored by Red Bull.
Its 11 members include Felix Baumgartner, who wowed the world in 2012 by jumping from a balloon floating 24.5 miles in the sky -- so high he had to wear a spacesuit and fell for more than four minutes before he pulled his 'chute.
And it all started nearly four decades ago because a snaggle-toothed skydiver and a handful of followers had the idea to jump off a cliff.
Why? To find adventure. To achieve the impossible. To break the rules.
Explore more about Carl Boenish's life and the early days of BASE jumping on CNN Films' "Sunshine Superman."