In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water (0 °C or 32 °F) for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of the ground material. A pioneering airborne electromagnetic survey in the Yukon Flats near Fort Yukon, Alaska, by the U.S. Geological Survey has yielded unprecedented images of the presence and absence of permafrost to depths of roughly 328 feet. The airborne survey captured images of permafrost over a substantially larger area, and with greater data density, than has been previously achieved using sparse boreholes and ground-based geophysics.