The floating septic tank reminds me of a story that never fails to make me laugh when I think about it.
My buddy's dad runs a hog farm in northern Indiana. He bought a rail car to use as a manure holding tank. He hired a guy to bury the car for him using some piece of heavy machinery the 'guy' had. I think this was in the fall of the year.
The next spring they got alot of rain, not atypical.
The rail car started to float, just like you warned on the plastic septic tanks. I went back there to look at it with my buddy the day after I heard the story. The front third or so of the rail car is sticking up out of the ground behind one of the hog barns. Man oh man is that ever a mess that I'm glad I don't have to deal with.