I'd get underneath and take a nice long look all over the under carriage and get a sense of how big a problem you have. Is it that one spot or more. Knock on wood that's the only spot! Then you can begin your best course of fixing it.
It hasn't caved in so that's a good sign. There's a thin layer of metal on the outside, and a thicker channel of metal underneath that has the nut for the body bolt.
But that does look like some bad rust.