There is a chance that air diffuse from the ground water get trapped in one of
the high spot, aka bubble, on the liner. Overtime, the bubble enlarged.. If you
slope the bottom of the liner enough, the air would not get trapped.

Richard Holub wrote:

Now for my question...I was planning of keeping the shallow side of the pond
at 4 ft. and the deeper side at 5 ft. I have read that if a liner pond is
constructed directly on the top of a wet land, the liner will bubble up.