Main Drain Failure?

My pool has started losing water (about 4 inches a day (inground 16x36). I am watching to see where it ends, but there has developed over the last couple years a large stain around the main drain (see pics). This is not dirt and the surface is as rough as it looks (ie--very rough). The size of the stain has been growing each year. Has anyone seen this before? Is this main drain failure, or more likely a leak in the vinyl near the main drain?

pictures:
from http://www.flickr.com/photos/zyzyx/s...57630028424298

https://lh4.googleusercontent.com/-L.../s800/pool.jpg

https://lh3.googleusercontent.com/-6...%2520algae.jpg

As above, but with enhanced contrast:
https://lh6.googleusercontent.com/-S...rized%2529.jpg

Detail
https://lh6.googleusercontent.com/-h...rized%2529.jpg

I can't tell for sure, but it looks like you have a badly wrinkled area of the liner (possibly from a leak eroding the surface UNDER the liner) coupled mustard algae that's accumulated in the wrinkles.

Mustard algae will brush up, at least somewhat, into a greenish or yellowish cloud. But, the only way I know to leak-check a location like that is with SCUBA. Some guys can hold their breath -- but you have to be VERY still to check leaks in a location like that, and even those who are good breath holders usually can't be that still.

Thanks for the info. At the current rate of drainage, I shouldn't have to hold my breath at all in a few days.

You might not want to let that happen by itself -- I'd call a pool builder or service guy. If it's leaking like you say, you're going to have to drain it and repair it anyhow -- and draining liner pools is pretty much a professionals-only task (yes, some amateurs succeed, but more end up destroying their liner!).

And . . . that amount of water leaking through can substantially increase the difficulty of repairs to the subsurface.