Weird spot on liner

[RMoss]

Any ideas what this could be? Haven't had a chance to get in the pool to investigate it up close yet, but it has me worried. It's about 4 inches across, I'd say. From outside the pool, it *looks* like the liner may be a bit wavy there, but not sure about that.

 

[elwood58]

It almost looks like somebody dropped one of the big chlorine pucks in the pool, and it sat there for a while.