Is it illegal to explore deactivated Minuteman II silos?

It would be tresping unless you got permission to be there. And tresping on federal property is always a big deal. If you do decide to do this anyway please be careful. Let someone know where you are going and when you will be back. Really will not know the shape of the building inside until you are there. Sometimes floors are rotten in empty buildings, and there can be a lot of broken gl.