The Kola Superdeep Borehole reached temperatures of 500 degrees at the bottom. If you could dig down to where the temperature reaches just over 212 degrees, would you be able to run a steam engine by putting water in the hole and letting it boil to create steam to power the engine. As long as the heat remained the same and was able to properly run the steam engine, it would seem like a very renewable energy source as the water could be collected and dumped back down the hole to repeat the process indefinitely. Kola Superdeep Borehole, https://archive.is/0zuzE Steam engines, https://www.mpoweruk.com/steam_turbines.htm