for my oil-fired boiler, the fuel oil tank in the basement has the see-through
plastic fuel gauge on top. It's cylindrical, vertical and marked off in
quarters. Approximately the bottom quarter of it went dark for some odd
reason, as if stained from the inside. The boiler still works the same as
usual. It was a very rainy year. The tank was filled last spring but I don't
think that this problem appeared right after that, but I'm not 100% sure of that.

Any ideas what did it?

photo he http://tinypic.com/r/34shxjq/5