Help - I have a vinyl IG will sand floor. the pool is in its' 5th season, at the end of last summer I discovered two spots that feel like small craters in the slooped sides of the pool. The are on opposite sides of the deep end. At first the were about the diameter of a golf ball. Noe they are about the size of a baseball and there is a third crater. What's going on and what can I do?