Gordon, I have never buried my tractor, but I have gotten the small one (a 2wd turf tire 27hp yanmar) stuck many many times, in fact it is one of the reasons that I ended up by a second, bigger machine (a JD5410, 4WD).
It doesn't take much to stick a 2wd tractor with turf tires in the mud, and I was always able to pull it free with my pickup. (or now wth my big tractor)
I have had a few scares with my JD5410, where the ground just kept sinking around my tractor, but even with the mud up/over the axes it kept on going thru like a trooper...I try to aviod situations like that because I know what an ordeal it is going to be to free a 10,000lb tractor from mud up and over the axels...that will require some heavy (read expensive) equipment, not to mention the embarassment...
I have a picture I'd love to be able to share with you guys if I ever get my scanner hooked up again...its a farmer, sitting on a JD tractor (not sure what kind, but its bigger than mine, has large-dual real wheels) with mud all the way up to the seat on all four sides (you can just see the tops of the 4 rear wheels)...the caption is "Every farmers worst nightmare". I love to look at it, makes me laugh and cringe at the same time....
Good luck.