Ballistic missile submarines (SSBNs) carry submarine-launched ballistic missiles (SLBMs), which are large, long-range missiles armed with multiple nuclear warheads. The SSBNs' basic mission is to remain hidden at sea with their SLBMs, so as to deter a nuclear attack on the United States by another country. Navy SSBNs form one leg of the U.S. strategic nuclear deterrent force, or "triad," which also includes land-based intercontinental ballistic missiles (ICBMs) and land-based long-range bombers.