In the last 49 years, U.S. SSBNs (nuclear powered ballistic missile submarine) have made 3,839 deployments (gone to sea for 11-12 weeks at a time). Recently, the current class of U.S. SSBNs, the Ohios, completed their 1,000th deployment. Each SSBN has two crews, who alternate in taking the boat to sea. Thus the subs spend most of their time at sea, where they are nearly impossible to find and destroy. Thus the SSBNs provide the ultimate retaliatory weapon to dissuade any enemy from trying to launch a surprise nuclear attack, with the idea of wiping out land based missiles before they can be launched. Given what we know now of the effects, on the planetary environment, of a large scale use of nuclear weapons, such a "first strike" is highly unlikely.
The U.S. now uses Trident II (D-5), three stage ballistic missiles, costing $47 million each. The nuclear warheads cost extra. The 58.5 ton Trident II equips the 18 Ohio class SSBNs. The missile has a range of 7,400 kilometers and is more accurate than the Trident I replaced. The Trident II can deliver up to five warheads. The missile entered service in 1990, while Ohio SSBN boats were still being built (the first one entered service in 1982, while the last one entered service in 1997). Each Ohio SSBN carries 24 missiles (120 warheads.) Eighteen Ohio class SSBNs were built, and fourteen are still in service, carrying half the United States's nuclear warheads. Another four were converted to carry cruise missiles and commandos.
The 15,000 ton Ohios is the latest class of U.S. SSBNs. The first was the five, 6,000 ton George Washington class, which was basically a SSN design that was enlarged to add the missile compartment (for 16 Polaris missiles.) The first of these entered service in 1960 and was soon joined by five of the 6,900 ton Ethan Allen class, which was designed from the start as an SSBN. These entered service in the early 1960s. Basically, this was an improved George Washington class. Next came nine, 7,200 ton Lafayette class boats, with the first entering service in 1963, and the last one decommissioned in 1994. The next two classes (James Madison and Benjamin Franklin) were similar, with incremental improvements. The last of these was decommissioned in 2002, after over 30 years of service, leaving just the Ohios. The incremental improvements were not trivial. The Benjamin Franklins had much quieter machinery, better electronics and enough room to handle the Trident 1 missile.