It's not "really" a secret - anyone in the area of Palmyra who knows baseball probably knows of Jim, and knows almost exactly where it comes from. I think what keeps this secret not well known is that it isn't exactly a gold mine - there is only a single buyer for the mud, and the sales total to about $20k/yr, with a lot of manual labor collecting and filtering the mud. It's not worth it for anyone to try to scoop his mud source.