You&#x27;d probably need to make it very slightly more dense than water, or it may get trapped at the surface in an underground cave.  This means it won&#x27;t surface by itself once it leaves the cave system, and as water blocks RF pretty effectively, you&#x27;d not get a signal.  To make it work, you&#x27;d need to add some additional mechanism to allow it to become more buoyant over time - perhaps a slow chemical reaction creating gas?