> What I don't get is why someone would play an elaborate prank, knowing that they probably won't be around to watch people's faces who try to find the treasure.

Because it's funny. I think you are right, the stone was probably on the surface or the tree, after someone collected the treasure they dropped the stone down there. thinking, wouldn't it be funny if someone dug up the stone and kept digging? Those idiots will waste their time while we spend all this loot on girls and booze!