Silicon has attracted great interest as a platform for both linear and nonlinear integrated photonics for over 15 years. While its primary applications have been in the telecom window (near 1.5 μm), the capability of exploiting its full transparency window to 8 μm in the mid-IR is highly attractive, since this will open it up to entirely new applications in fields such as spectroscopy, chemical and biological sensing, and free-space communications. However, while silicon-on-insulator has shown great promise just beyond the telecommunications window [to the shortwave IR band (2.5 μm)], its wavelength range has been limited to < 4 μm by absorption in the silica cladding layer. Here, we demonstrate octave-spanning supercontinuum generation in silicon, covering a continuous spectral range from 1.9 to beyond 6 μm in dispersion-engineered silicon-on-sapphire (SOS) nanowires. This represents both the widest spectrum and longest wavelength generated to date in any silicon platform, and establishes SOS as a promising new platform for integrated nonlinear photonics in the mid-IR.