First near sink: A trailer park in Florida, 1967. I think I had seen a movie where a woman was stuck in over her waist in quicksand, and tried to get near the mud in the bottom of a ditch, but slipped so that only my fingertips held the asphalt at the lip of the ditch to keep me from what I just knew was certain doom.

I held on and screamed until my father came and lifted me out.
For some reason, the memory of that memory still stands out today. (and I was only 4 years old at the time)
Five years later, Sunday morning, my brother, sister, and I went down to the nearby spring and played in clay that was knee-deep on my 9-year-old self. We had a BLAST, but were covered head-to-toe in mud. (a mud-slinging fight having broken out) My father called us in to get ready for church. As we got finished washing up and changing, we were put out the door to wait. I had had such fun at the spring that I went back to just throw mudballs at the fence. While that was likely an okay plan for just me, when my younger sibs got out there, the mud-slinging fight restarted and by the time my father was ready for chruch, he left our house to find all of his kids recovered with mud. I (happily!) have rarely gotten such a switching!
It was the Little Colorado River that was likely my best sink yet. I was in a mix of clay and sand up to my elbows before floating. (With any movement rippling the ground for several feet in all directions) With a good effort, and some time, I was able to get all the way down to chin-deep before I was just too tired to keep struggling, at which point I bobbed right back up to elbow-deep.
The Virgin River Gorge had promise, with some clay mud along that river being waist deep (that I found!) and STICKY! I couldn't find any deeper though, and even with the mud being sticky, waist deep is not nearly deep enough to stop me. At least the river had a nice flow, so cleaning up was easy.

I still want to hit some of the quicksandy tidal areas near England or Northern France. I would love to buy my own old gravel pit (and honestly have not yet given up on that idea) but no money just yet. Belive-it-or-not, I would like to head down to Trinidad and poke a toe or two into the Pitch Lake.

(that, ya need to be careful about, as humans don't float in tar)
Yerk