Four years ago we bought a house with a pool which was installed in 1966. The pool has clearly been resurfaced, and we know that a tree from the neighbor's yard fell into the pool in 1996 which damaged the bottom of the pool and these areas were patched. The patches are intact, but clearly visible and are becoming less cosmetic as time passes.

My question concerns a black spot on the bottom of the pool which is slowly growing. When we first noticed it, it appeared something was on the bottom of the pool. When investigated, it was discovered to be in the gunite. Over the last two years it have grown from dime size to golf ball size. They are not removable by brushing.

It is currently raining, but we will post pictures when the weather allows.

Any ideas?