Gravitational waves theoretically effect themselves due to "self-gravity" http://arxiv.org/abs/0704.1149 I thought this was interesting. From a layman's perspective, a wave is a bundle of energy distributed through the medium it's traveling through. In the case of gravity waves (which themselves are space-time distortions), their presence within space-time affects the curvature of space-time due to their energy. So gravity waves are unique, since their presence effects their own boundary conditions. Now consider a spherical distortion with such great energy-density that it bounds itself as a mini-black hole and forms a stable standing wave. Is this possible? I'd imagine the math would be horribly complex, as the boundary conditions of the standing wave are dynamic and coordinated with the standing wave's structure, but maybe something special can be found, such as a discrete spectrum of frequencies and energies of stable solutions.