I think these are very old deposits of water trapped in the clay. They hit one in my last pond that filled the basin of the 1-acre pond with 3' of water in one large swoosh. The water turned blue like pictures from the Caribbean. That pond does not leak. We are bone dry in the mist of a very bad drought. We installed a drain the pond bottom that I though would drain this water away, but it seems to be holding back in pockets in the clay. Very interesting geology.
