  We describe global, 3D, time-dependent, non-radiative, general-relativistic,
magnetohydrodynamic simulations of accreting black holes (BHs). The simulations
are designed to transport a large amount of magnetic flux to the center, more
than the accreting gas can force into the BH. The excess magnetic flux remains
outside the BH, impedes accretion, and leads to a magnetically arrested disc.
We find powerful outflows. For a BH with spin parameter a = 0.5, the efficiency
with which the accretion system generates outflowing energy in jets and winds
is eta ~ 30%. For a = 0.99, we find eta ~ 140%, which means that more energy
flows out of the BH than flows in. The only way this can happen is by
extracting spin energy from the BH. Thus the a = 0.99 simulation represents an
unambiguous demonstration, within an astrophysically plausible scenario, of the
extraction of net energy from a spinning BH via the Penrose-Blandford-Znajek
mechanism. We suggest that magnetically arrested accretion might explain
observations of active galactic nuclei with apparent eta ~ few x 100%.
