Anya cava

If we were to surround a rotating black hole with a mirror and shoot a laser beam inside it, the beam would get exponentially more and more powerful by stealing energy from the hole via superradiant scattering. It could eventually overpower the mirror and result in a powerful explosion on par with a supernova. We could prevent these black hole bombs from going off by harvesting some of this energy. Using this type of energy we can theoretically power civilization for millions if not billions of years. In fact, these black hole civilizations may be the last home for any species in a dying universe.

The amount of time it would take Mount Rushmore to erode into unrecognizability (assuming that it is not maintained) is approximately 7.2 million years.