Mine was swimming over a bottom that I thought was reasonably solid,only to sink into something that was sort of like dark chocolate pudding...you could feel it cling to your legs, and the first movement to get out cause a complete black out...zero vis, zero light..and the bottom of it was so thick you could not move your legs to kick..even though a lot of year ago, that feeling of complete lack of control and being trapped has stayed with me. Since then, I've been thru a lot of situations where I started moving towards panic, and all I have to do is tell myself "lets just take a second and relax, we really don't want to go back to that place."
I think my own actions scared me more that the situation... I had a ton of air, and while the stuff was thick, I knew it could not be more that 6 or 8 ft deep. But I panicked and completely opened the fill bottle on my Fenzy vest (instantly adding just under 70 lbs of lift)...it took seconds to move at all, then slowly moved and then I shot to the surface. Don't know if I exhaled, inhaled, and a lot of really bad things could have happened in those few seconds. I was fine, but if you have ever been thru an adrenaline rush like that, you come down really hard. I went to shore, refilled the Fenzy fill bottle and went back to the area and sank back down in the stuff. Bet it took me 10 minutes to actually relax... and another 10 to figure out how to get out (kicking is not the answer, as you tend to sink deeper)...
I believe, had I not gone back, I would have given up diving.
Later I would take students out there, never saw anyone that did not panic the first time, even when told what they were going to experience...