I saw it a few times in the 1980s...A part of the Wild West stunt show, which no longer exists. The tour is still there.
It's basically a rubber bladder or something like that over a pit with a small closable opening in it and sand over the top. The person standing on it would slip down through the slit and disappear under the sand, then traveled through an underground tunnel and re-appearing elsewhere later in the show. They may have been lowered by a lifting/lowering device.
Very hard to find footage from then, as most of the videos I see are of the Florida version of the park, where they feature falling into a well rather than sinking in QS. Not impossible to find, but hard to find good quality.
Try these:
https://youtu.be/xv8Kpc_z1Bg?t=4m32shttps://youtu.be/4G6GqRRMDJM?t=1m58s.
"Life is tough. It's tougher if you're stupid." - John Wayne