To observe the faintest objects in the Universe astronomers require the darkest skies. In the infrared, sensitivities are limited by the thermal emission from the atmosphere and the telescope. By placing a telesocpe in Antarctica, and exploiting the reduced thermal emission and the natural absence of strong airglow emission between 2.3 and 2.5 microns, we can minimize the sky brightness. In this paper, and in an accompanying paper by Nguyen et al. 1996, we provide t"cosmological window". At 2.4 microns the sky flux can be as low as 50 mu-Jy/square arcsecond, up to orders of magnitude lower than the corresponding flux at temperate observatories. We also show that substantial reductions in the background can be achieved throughout the 2.9 to 4.1 micron region.