Subterranean clover (Trifolium subterraneum), the name often shortened to "sub-clover", is a species native to northwestern Europe.  It is now widespread in many parts of the world, and is one of the most commonly grown forage crops in Australia.  Sub-clover is extremely high in protein value, highly palatable to livestock, fixes nitrogen into the soil, and withstands heavy grazing (which actually promotes the sub-clover stand).  The University of California and Australian researchers worked for decades together to find correct varietal types and rhizobium (soil bacteria capable of forming symbiotic nodules on the roots of legumes and then fix atmospheric nitrogen into the soil) that is needed to fit the various micro-climates.  Nowadays when one purchases sub-clover seed from the local farm supply store one actually gets a bag of pre-coated seed that has the rhizobium pel-coated onto the surface of each seed.  Much of the mentioned field research was done at HREC by UC Davis agronomists.

Oh, the plant is given its name because it normally blooms close to the ground, and as the seed-burr forms the plant actually buries the seed into and sometimes below the surface of the soil, thus the name "subterranean".