Those two guys are going to tumble for what seems to be a lifetime, until they finally hit bottom with sand in every orifice of their miserable bodies. I speak from experience.
I calculated I covered 500' in 14 seconds flat doing that when I was a teen-ager. That's ~35 ft/second or around 24 MPH. Fastest I ever went without a vehicle. :-)
Years ago when I was about 20, I climbed to the top of sand dune south of South Haven. When it was time to go back down, I started bounding downward covering about 30 feet with each step. Somehow I was able to stop before I reached the bottom. It was an incredible feeling, but I’ve never done it since. Now I’m too old (54) to do something that stupid.