Deep borehole disposal is the concept of disposing high-level radioactive waste from nuclear reactors in extremely deep boreholes instead of in more traditional deep geological repositories that are excavated like mines. Deep borehole disposal seeks to place the waste as much as 5 kilometres (3.1 mi) beneath the surface of the Earth and relies primarily on the thickness of the natural geological barrier to safely isolate the waste from the biosphere for a very long period of time so that it should not pose a threat to man and the environment. The concept was originally developed in the 1970s, but recently a proposal for a first experimental borehole has been proposed by a consortium headed by Sandia National Laboratories.