Short describe about my concept of sinking xDDDD:
I like to find different hard and non passable places, like as mud fields, marshes, bogs, swamps, quicksands, dried lakes or ponds, etc. and try to go through it. Especially cool for me, if it have to much deep - till chest or neck or even "bottomless". We love to do it with my girlfriend. It's those, what turn on for us - going through, sinking, stuck, and whatever go forward... Trying for get out, helping each other and still going and crawling...
We like to do it naked and barefeet - no clothes - more simple to recovery! Sex in mud - yes!!! Sure!
If place is quite and lone - sure!!! Others we are dressed in minimal clothes - no shoes, and beach variant or shorts, swimsuits... Also we like barefooting - maybe as part of our deviations, but it so cool, funny and awesome, especially when rainy days and mud around!!! But it's other story. We don't like: artificial places (as bathroom mud, "backyard" quicksand, cork and sawdust in movies), going under with head (but can be exclusions), ridiculous suits or cosplay, places with too much water...
Off-topic!!! Sorry!!!
Also I must say dedicated for cow manure (I know - it's forbidden here, but let it will be off-topic, I just must to explain in couple words for...) - yes, it's cool! I insist - it's not scat and not copro!!!! Speech is only for cow manure! No any others not acceptable for us!!! It's part of farm mudding, it can be deep and looks as real bog or quicksand, it can be sucking, it can be salubrious! We don't like just rolling in it or jump into pile or damp pit. We going trough if it damp huge territory and look as bog... Ok, that is all about here!!! If you want - i can tell more with admin permission or just in priv messages...
... that is news to me. I will always think of you whenever I see a cow pie. 
I have stepped in it with boots on. It's like rotting grass. To each his own and more power to you.