There are mine communication devices that can punch a signal through a few hundred meters of solid rock.  Here&#x27;s a simple one.[1]  VLF can get through, with enough power, a simple signal, and a smart receiver.[2]<p>There&#x27;s been a lot of work on &quot;down-hole communication&quot; for oil and gas drilling.  Modern drilling involves com links to the sharp end, with info coming up and commands going down. Some of the techniques work through the drill string, but some are VLF radios.<p>There are also high-powered ultrasonic pingers, with a range of several kilometers in water.[3]<p>If you could get info for the first kilometer or two, you&#x27;d at least know where to look for the rest of the path.<p>[1] <a href="http:&#x2F;&#x2F;radiolocation.tripod.com&#x2F;" rel="nofollow">http:&#x2F;&#x2F;radiolocation.tripod.com&#x2F;</a>
[2] <a href="http:&#x2F;&#x2F;onlinelibrary.wiley.com&#x2F;doi&#x2F;10.1029&#x2F;2010RS004378&#x2F;full" rel="nofollow">http:&#x2F;&#x2F;onlinelibrary.wiley.com&#x2F;doi&#x2F;10.1029&#x2F;2010RS004378&#x2F;full</a>
[3] <a href="http:&#x2F;&#x2F;www.sonotronics.com&#x2F;?page_id=96" rel="nofollow">http:&#x2F;&#x2F;www.sonotronics.com&#x2F;?page_id=96</a>