I usually just go for it, because I have been in stuff that was like milkshake, and still floated! In fact, I even pulled myself under so deep in that stuff that when I reached straight up, my fingers still couldn't touch the surface, and there was more soft mud beneath my feet! I then just climbed my way back to the surface. That has made me confident about buoyancy in deep mire.
But, however, there was one spot where a few feet of thick peat was hiding water underneath, but it was a small pit, and after vigorous struggling for a while, I crawled over the surface to solid ground a few feet away. Nothing like sinking into thick mud, when suddenly, at around crotch deep, it drops away from your feet, and they are dangling in water, which starts bubbling up around you, and you start sinking faster!
It was located on the north side of one of the isolated clumps of large willows in
this meadow. There were bones scattered around the meadow in the area. I don't know if it is still there, or if it has been grown over with grass, since I haven't been there since the late 80's.
How deep I go under with a lungful of air depends on how loose the mud is. The looser it is, the deeper I can go, because I can move more quickly through it for a round trip down and up. The thicker it is, the slower going, and therefore, the less distance beneath the surface I will go, although if it is able to stay open, I may go a bit further down, if it is the stuff that parts above my head when I push back up. When I use a breathing tube, I only go down as far as I am comfortable with, and make certain that the tube won't come out of my mouth while under, and that the other end doesn't go under, if it is a short tube. For a breathing hose, I also make certain that the other end isn't going to fall into water, although I tend to worry that something like a mouse could run into the hose, through it, and into my mouth while I am deep under the mud!
any thoughts 
, I could have actually taken those pictures.
