The existence of cosmic-rays with energies well above 10^20 electron volts (16 Joules) is a long-standing mystery; producing them requires either enormously powerful astrophysical accelerators, or something very exotic. One way to shed light on cosmic-rays accelerators is to search for their signal in another channel - neutrinos! The IceCube detector has observed a diffuse flux of astrophysical neutrinos, extending to energies well above 1 PeV (10^15 eV). This flux has been observed by multiple analyses using independent datasets, with high significance. This flux is near the maximum predicted ("Waxman-Bahcall bound") from astrophysical accelerators, but, surprisingly, we have not yet been able to associate this flux with any distinct sources, or classes of astrophysical sources. The most popular source models, involving gamma-ray bursts, active galactic nuclei (galactic centers containing supermassive black holes), and even star-forming galaxies are disfavored to varying degrees, opening up room for a wide variety of speculation. In this talk, I will discuss the IceCube results in the context of cosmic-ray physics and give the status of different classes of source models. I will conclude by discussing some possible future experiments.