Eternal Black Hole -- from Eric Weisstein's World of Physics

A massless black hole which is a stable topological structure held together by the nonlinearity of its gravitational field.

Anninos, P.; Hobill, D.; Seidel, E.; Smarr, L.; and Suen, W. M. "Collision of 2 Black Holes." Phys. Rev. Let. 71, 2851-2854, 1993.

Anninos, P.; Bernstein, D.; Brandt, S. R.; Hobill, D.; Seidel, E.; et al. "Dynamics of Black Hole Apparent Horizons." Phys. Rev. D 50, 3801-3815, 1994.