Many (but not all) bacteria produce gas as a by-product of their metabolism, so does the process of fermentation by some fungi. The combination of the two can produce enough gas to raise a drowned body to the surface. However, should a drowned body sink well below the thermocline as in a deep lake, the temperature is so low at the bottom that the processes of decay are slowed, or even stopped, and the body will not rise to the surface. Such was the case with the tragic death of Donald Campbell when his jet-powered hydroplane flew apart on Conniston Water. His body was only fairly recently discovered and raised for proper burial. I won't go into details here but the one diver desccribed Campbell's body as being like a large piece of soap!!
47 mths ago