Dark spots on new Teak and Holly flooring

[pwmullins]

Back in 2012 I had some major repairs made on my Endeavour 42 after a grounding. At that time the fuel tanks and water tanks were removed and replaced. In doing so, the boat yard destroyed the T&H flooring in the main saloon. Everything has been fine until the last several months when a dark spot about 12" in diameter has appeared in the middle of the flooring. The flooring covers the forward water tank. I purposely keep this tank dry except when I go cruising which I haven't had the opportunity to do this past year. Does anyone have any idea what's causing the spots?