Bubble under EPDM

[BarbO]

My lined pond, 500 gal, had a bubble of something on one of the sides recently. When I touched it with my pond net it deflated and went away. The bubble was about 6" x20". Is this gas from the ground perhaps? Not water or it wouldn't have deflated so fast with slight pressure on it. Have never seen such a thing before, my liner is 45 ml. thick, no holes or damage at all. What do you think?

 

[GBBUDD]

Yes we call it a hippo because it looks like a hippos back poking up. It can be from gas by more times then not it is from water . WHEN YOU PUSHED ON IT THAT IS PRESSURE AND IT COULD DISPERSE THE WATER MORE EVENLY ACROSS THE POND UNDER YOUR LINER.
The way to solve this is to get a hose or a pipe down under the liner to give the water or even the gas a place to escape rubber traps it and has no where to go . Giving it a open channel /pipe this will give a path of least resistance