Back in the 1870s the 18-Something or Other Mining Law was passed in Montana that you could buy a piece of land to build a house or pasture your horse, but the mineral rights belonged to the Anaconda Mining Company. The law is still in effect today. You buy a house, knowing that a mining company could file a claim and lay waste to your property if there was anything in the ground they wanted. Not all property was included, and I don't know why, but over the years I bought three properties, all with easements for mining.
The law had become so one sided and untenable, that to make a point, an environmental group filed a mining claim on the 9th hole of some golf course. Now to keep a mining claim, you must actively do something to show purpose, so the group threatened to build a cabin on the area as a base for starting exploration. It was all entirely legal and yes, some feathers were ruffled, but they never built the cabin, because they just wanted to point something out. Nothing about the law changed, but legally, they could have followed through with the destruction.
Make sure you clearly understand the title to the property before you decide to sign. You may not own what you think you bought.