I recently purchased an old house for my family which has a pool. The pool has dark water and a lot of debris. I heard that there is a risk of a pool popping out of the ground if you drain it. How do I find the dead man line (hydrostatic pressure line)?

My Hayward 60 sq foot DE filter has too much pressure 32 psi overnight from 12 psi the day before. The filter has 2 seasons on it. Here is what I did pool was green so I shocked it and vacummed alage straight to the street using the waste port as not to run it thru filter grids. I got the pool blue again a bit cloudy but blue. Next I backwashed filter and removed grid assy and washed off all remaining DE.(I did not take out each grid just the assy). I then installed assy and rechared with 12 cups of DE pressure was 11psi then. I decided to let pump run overnight to maybe clear the water more but this morning I went out and psi was 32 psi. How can it go so high in one night with a fresh DE recharge? I dont know if this matters but I have a salt water system but it is low on salt and I shut it off till I get done with the cleanup at witch time I will add salt as required. I just went out and put the valve on recirculate and the pump is putting out plenty pressure. I switched from skimmers to drain and in between and lots of water shots from the jets. So I believe the problem is in the filter and not the return lines.