there could be much smaller ‘mini’ black holes with the mass of say, a mountain. A mountain-sized black hole would give off X-rays and gamma rays at a rate of about ten million megawatts, enough to power the entire world’s electricity supply. It wouldn’t be easy, however, to harness a mini black hole. You couldn’t keep it in a power station, because it would drop through the floor and end up at the centre of the earth. If we had such a black hole, about the only way to keep hold of it would be to have it in orbit around the earth.
