It probably wasn't visible from ground level. If it's five feet below the surface but 30 feet from shore, then your line of sight would pass through a bit more than 30 feet of water, and the pond is muddy enough that it's partly obscured even from directly above. If no one happened to swim or boat directly over it, and with no tall buildings nearby, there may have been no opportunity to see it.

(I've noticed something similar flying over the Great Lakes. Looking down from the window of a descending plane, you can clearly make out lakefloor contours that are invisible from shore, even with the relatively clear water.)