This is a group of Canadian climbers visiting Joshua Tree. Some of them used a safety leash and there was at least one person who went across without any safety measures. The potential fall here is 65 feet onto solid rocks and boulders.
Short History: The very first time I saw something like this was in 1970, in Yosemite's Camp 4/ climbers campground. It consisted of a chain, 30 feet long, hung between two trees about 3 feet off the ground. Jim Bridwell , a Yosemite icon, and his buddies used to walk on it as the chain swung left and right like crazy. Everytime my buddies and I went there, we tried it too.
At least a decade later, the chain had turned into a one-inch-wide nylon webbing stretched between two trees. It was now called "Slackline."
Then, people started to increase the hight, 10 feet, 20 feet,... hung between two rock formations.
The latest, as far as I know: A famous Yosemite climber, Dean Potter, without any safety measure, has walked the slackline stretched between Lost Arrow Spire and Upper Yosemite Falls Wall, a potential fall of 2000 feet.