The heat is generated by natural nuclear fission of uranium and...

The heat is generated by natural nuclear fission of uranium and thorium far below the surface of the earth. (One wonders if the Greens will be alarmed about the possibility of these natural radioactive sources coming to the surface in ages to come, as they are about artificially buried nuclear waste. Perhaps Peter Garrett can organize an on-site singalong, just in case local aborigines wander too close and need to be warned.)