Like so many others here, I love sinking because I get close to nature, love the pressure/feel of the mud around me, especially on my skin, holding me, taking a fateful step/jump in/onto a patch of ground that I know is going to suck me down deep! I love the way it heaves, pulsates, undulates, quivers, sometimes burps and farts, as if it is alive, sucking down its meal, namely me!

I love the struggle in it, "trying" to escape, but every movement I make only makes it undulate more, and suck me down even faster, loving the sounds and the smell of the swamp gas escaping, getting more and more trapped! I love getting sucked completely under, with it closing over my head, trying to suffocate me, but fails as I work myself back to the surface to breathe again, and then go under again. Occasionally, I love breathing through a hose while totally submerged, my entire body wrapped within the doughy confines of the pulsating bog. I love struggling horizontally on the surface of the bog, pretending to have fallen and unable to get back up, and slowly sinking, stomach down, pretending to be getting sucked down "to my death"! While moving through looser mud feels really nice, and I enjoy sinking and struggling in it, it is the thicker, doughier stuff that I love best, since it offers the most resistance to movement, and more of the entrapment, while getting sucked down deeper and deeper!
I love organic (decayed plant material such as peat) mud (mainly the thick stuff) because it is low density, allowing me to go under easily. It also is the stuff that decays further, producing all that wonderful swamp gas that burps and farts as I get sucked down to my "death"! I love quicksand/silt because it can feel solid at first when you walk on it, but then gives way, like standing on a rubber membrane that keeps on settling down lower and lower under your feet as you sink down with it! The thick ooze your body is slowly being sucked into feels soft and yielding with slight movements, but tenses up like solid muscles around you when you try and pull yourself out hard, only to loosen again when you relax. I prefer quicksilt over quicksand, because of the finer grains which feels nicer, and it doesn't settle around you as quickly, especially if it has some clay mixed in with it. It also behaves the same way as quicksand, with the "muscular mire" tightening its grip when you try and pull too hard, and loosening when you relax, and settle in deeper (until you reach your buoyancy point). I love clay because it can be so sticky, and the right stuff can cling to you like tar, and the feel of it is more uniform than the other types of mud. Thick clay can also have a feel as if it is dry to the touch, even though it contains water, which makes it even more interesting. Perhaps due to its density, even looser clay (and silt as well) can exert pressure on you similar to organic mud of a much thicker mixture. But, the disadvantage of quicksilt/sand/clay, is that because it is mineral based, it is denser, making it more difficult to sink in deep. But, that doesn't keep me from trying!
I have a number of videos of me sinking at
viewtopic.php?f=19&t=2588 , and there are some more in the works.