Let me try a lot more literal angle, though I have only ever been down to my neck.
You are simultaneously supported, restrained, and squeezed. The restrained part is the most interesting of these three: depending on the thickness of what you are sinking into, it does let you move somewhat, but not too much. You can often easily move your feet around, but for example moving your leg forward while it is submerged is impossible in all but the wateriest of spots.
As you probably already know, it's also cold, wet and slimy. I kinda like the being covered in thick, heavy slime part too, it's an interesting experience. My body just feels different to normal, everything is slippery.
Herzeleid wrote:so far I've only ever managed to sink thigh deep, as there aren't many locations for sinking where I live.
Depending on how your mud is, you might want to give things other than sinking straight down a shot, eg. trying to sink kneeling or sitting down, or even just taking a bit of a mudbath. I had fun with this sort of thing when I didn't find a good enough spot, but YMMV. Not as good as sinking straight down, but still fun. Just be careful not to get too stuck.
