Electric Water Heater Leaking From Top

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On the lid of my water heater is a cold water inlet pipe and what looks like a freeze plug to the right of that; clockwise, there is the main electrical cable connection; on the far left is another freeze plug; inboard from that is the hot water outlet pipe . Directly behind that pipe is a 1" round hole in the surface of the lid with no freeze plug or cover of any kind; it's just an open hole. There is rusty water filling this hole and it is very slowly leaking over the edge of the hole,  across the lid to the nearest edge and running down the back of the water heater in a tiny stream about the width of a piece of string. What is going on and how do I stop it? Thanks!