Some rare cosmic rays pack an astonishing wallop, with energies prodigiously greater than particles in human-made accelerators like the Large Hadron Collider. Their sources are unknown, although scientists favor active galacti nuclei or gamma-ray bursts. If so, gamma-ray bursts should produce ultra-high-energy neutrinos, but scientists searching for these with IceCube, the giant neutrino telescope at the South Pole, have found exactly zero. The mystery deepens.