🚀 go-pugleaf

      Thermocouples may be a good idea here.  Most of the time, there's a
useful temperature difference between the surface and 50' underground.
An old well shaft should be a good source for a few hundred mA.  
Or you could drill your own small hole, which, for moderate depths, is not a
big deal.  Put all the electronics in a cylinder, lower it into the
shaft a short distance, plug the shaft with cement, and cover it over
with dirt, and, in a reasonably undeveloped area, it is not likely to
be bothered for centuries.  

      Underground antennas, just below the surface, are used in military
installations where blast-resistance is desired.  So even the antenna
can be underground, if you can store up enough power to get out despite
the losses.