There are two surfaces on which the Kerr metric appears to have singularities. The inner surface is the spherical event horizon, whereas the outer surface is an oblate spheroid. The ergosphere lies between these two surfaces; within this volume, the purely temporal component gtt is negative, i.e., acts like a purely spatial metric component. Consequently, particles within this ergosphere must co-rotate with the inner mass, if they are to retain their time-like character.

Since the ergosphere is outside the event horizon, it is still possible for objects in it to escape from the gravitational pull of the black hole.