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?

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As above, but with enhanced contrast:


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