  Fine-tuning generic but smooth spherically-symmetric initial data for general
relativity to the threshold of dynamical black hole formation creates
arbitrarily large curvatures, mediated by a universal self-similar solution
that acts as an intermediate attractor. For vacuum gravitational waves,
however, these critical phenomena have been elusive. We present, for the first
time, excellent agreement among three independent numerical simulations of this
collapse. Surprisingly, we find no universality, and observe approximate
self-similarity for some families of initial data but not for others.
