Homestake, which contains over 600km of tunnels, operated as a gold mine from 1876 until 2001. Scientific experiments have been carried out there before -- in 1965 it became home to the world's first solar neutrino detector, which was set up by the late Raymond Davis of the Brookhaven National Laboratory. Davis went on to share the 2002 Nobel prize in physics for this work.

The proposal for the new lab involves experiments at two levels -- one at about 1500m below the surface and another at around 2200m down. The intermediate level, which is where Davis set up his experiment, will involve the modification of an existing scientific site and the setting up of a number of new experimental chambers, while the deeper level will involve the conversion of caverns, boreholes and other structures previously used by the miners.

Many different types of experiment could be built at Homestake, including a number dedicated to studying the elusive neutrino. For example, physicists could build detectors to study an extremely rare nuclear process known as neutrinoless double beta decay, which, if real, would mean the neutrino is its own antiparticle and which would also permit researchers to work out the absolute mass of the neutrino. The mine might also host detectors to study the properties of neutrinos that have been sent some 1500km through the Earth from Fermilab near Chicago.

Other physics-based experiments at Homestake could include studies of proton decay or nuclear astrophysics, or lead to the development of next-generation gravitational wave detectors. Scientists working at the lab would also be able to study the Earth's crust, examine life forms that live in conditions of extreme heat and pressure, as well as improve technologies for sequestering greenhouse gases underground.

The proposal for the new lab has been put forward by a multi-institutional collaboration of researchers headed by Kevin Lesko, a physicist at the University of California Berkeley. It was chosen ahead of three other proposals by a 22-member panel of experts appointed by the NSF, and marks the culmination of a drawn-out selection process. Homestake was originally put forward as a potential host several years ago, but disagreements between Barrick Gold Corporation, the Toronto-based company that owns the mine, and the South Dakota state government and the NSF have held up progress. In 2003, Barrick turned off the pumps that were preventing the mine from flooding, prompting fears that the site would be unfit for experiments.

The Homestake collaboration is now set to receive some $5m per year for up to three years from the NSF for design work. Funding to actually build and operate the lab will need further approval and ultimately require the go ahead from Congress. If approval is forthcoming for the current design, the Homestake facility would be the largest and deepest underground lab in the world, surpassing existing labs in Italy, Japan and Canada. It is to be called the Sanford Underground Science and Engineering Laboratory in recognition of a gift of $70m provided by the bank owner Denny Sanford.