"rust" spot on bottom of pool

[Gillespie]

I have a Zodiac soft-side above ground pool. I installed it 5 years ago with a base of 4 inches of crusher dust, a tarp and 1.5" high-density foam on top. Last year I noticed a small (1/4" diameter) rust-cloured spot on the pool floor. (orangy-brown) It won't brush or scrub off. This year the spot seemed to grow a bit, and I've got another 5 or six smaller spots in the same general area (over a 16 sq ft area)
Someone said it looks like a nail is rusting underneath the pool. But this doesn't make sense...the rust wouldn't be able to penetrate the the floor, plus when I installed the pool I checked carefully to make sure there was no foreign material that might puncture the floor. Not to mention the spots are round and seem to be multiplying!
I'm worried that the disclouration is growing and that it may be corrosive to the point it will eat the vinyl.
Anyone ever seen this??

[lumberjack0070]

I have a small rust spot that just startred about 2 months ago just below the waer line on an inground gunnite pool . I can take one of those erasers and get most of it off, but it comes right back. Could this be some metal , such as re-bar that could be coming through the plaster? What would be the best way to find the problem? I am afraid if i dont do something soon the rust spot is going to keep growing. I take extremely good care of my pool and would appreciate your help. The plaster is Carribbean Blue in color,
thanks