In lightning bluff there is a square hole north of the tracks. There is…

In lightning bluff there is a square hole north of the tracks. There is a shiny/glittery spot on the right side of the square. I want to find out what is in there! I've tried irrigation and dynamite. I also brought the rabbit over, but nothing reveals what's under there. When you drag a person over to it, the grab bag icon shows up, so I know there must be something there. Anyone figured this out yet?