The deaths of extreme athlete Dean Potter and his friend Graham Hunt in Yosemite National Park brought renewed attention to the risks of BASE jumping and its more dangerous offshoot, wingsuit flying.

WHAT IS BASE JUMPING?

The term BASE jumping was coined by one of the sport's pioneers, Carl Boenish, in the early 1980s. It's an acronym for the platforms parachutists leap from: buildings, antennas, spans (bridges) and Earth. Jumpers launch into the air and deploy their parachute. BASE jumpers practice skydiving before taking their pursuits to earthly launch pads.

While people have parachuted off objects for hundreds of years and two men jumped off Yosemite's El Capitan in 1966, the modern form of the sport didn't really take off until the late 1970s in the park, said Tom Aiello, who runs the Snake River BASE Academy in Twin Falls, Idaho.

WHAT IS A WINGSUIT?

Wingsuits have been used in skydiving and BASE jumping to transform a free fall into flight. The suit has fabric stitched between the arms and body and between the legs, so when a jumper spreads their arms and legs, they can stay aloft longer. They can control their path with subtle body movements. Small changes can have big consequences when flying close to cliffs or near trees.

While parachutists experimented with wings as early as the 1930s and there was another wave in the 1960s, Aiello said the original suits "were widely and accurately referred to as death traps."

Commercial wingsuits were developed in Europe in the late 1990s and several companies now make performance suits selling for $700-$1,800 that are built to optimize speed or control. Speeds can exceed 100 mph.

HOW MANY PEOPLE HAVE BASE JUMPED?

Official records aren't kept of BASE jumps, but Aiello estimates that 10,000 people around the world have jumped at some point. The number of weekly BASE jumpers is probably about 1,000 and another 1,500 people may jump a couple times a year.

Aiello stopped counting his BASE jumps after his 1,000th in 2003. He jumps daily from the 486-foot high Perrine Bridge on his way to work after dropping his kids at school.

Most regular BASE jumpers have probably flown in a wingsuit at some point, but only a small number regularly jump in one, Aiello said.

Of that number, only 20 to 50 people in the world practice the type of "terrain flights" Potter did, flying through canyons and just inches from rock walls and treetops. The small number of people practicing the sport at that level is surpassed by the popularity of heart-pounding videos of the death-defying flights that have been viewed millions of times.

IS BASE JUMPING LEGAL?

At places like Perrine Bridge, jumping is allowed, but other sites, including private property and national parks, are off-limits. Yosemite experiment briefly with a permit system in 1980 but quickly scrapped it and made the activity illegal.

In 1999, a woman leapt from El Capitan in a public protest of the policy. People below watched in horror when her borrowed chute didn't open and she plunged to her death. Witnesses said she borrowed the parachute because she didn't want hers confiscated by rangers.

HOW MANY PEOPLE DIE BASE JUMPING EACH YEAR?

Many BASE jumpers have died, though the exact number is unknown because official records aren't kept. A European website that follows the sport has a tally of known deaths around the world. It listed 22 jumpers killed last year and 24 the year before.

High-profile deaths include Boenish, who died during a jump at Troll Wall in Norway in 1984, two days after setting a record for the highest jump. He was 43, the same age as Potter.