Wald and Iyer [161] proved this by explicitly constructing a family of angularly anisotropic slices in Schwarzschild
spacetime which approach arbitrarily close to
yet contain no apparent horizons. However, Schnetter and
Krishnan [136] have recently studied the behavior of apparent horizons in various anisotropic slices in Schwarzschild and
Vaidya spacetimes, finding that the Wald and Iyer behavior seems to be rare.