- Stores and launches Nuclear and Thermonuclear Devices.
Note that each of these devices has a maximum Range (12 for Nuclear Devices and 15 for Thermonuclear Devices), so where exactly you decide to put a Missile Silo matters quite a bit. If you put it in the middle of your empire you won't be able to hit anything important, so consider where your conflicts will force you to use weapons of mass destruction, and build a Missile Silo in your nearest city's territory.
One interesting trick that some players have been known to use in the past with regards as to missile silos relates to territory captured from an enemy (usually A.I.) player. First they first swap the tiles from this territory that they find ideal for silo placement on to their nearest non-captured cities. Then they build the silos, or alternatively leave Combat Engineers in place to build them later. Finally they return the city associated with the rest of the territory to the enemy civ it originally was sized from (gaining diplomatic creditability with A.I. players in the process is a bonus). The swapped tiles will remain part of your empire, in effect creating nuclear armed enclaves within the enemy's territory. This means that in theory at least a player can place silos in locations that are otherwise unavailable, in the process complicating the military options of other players.
Like everything else rocketry, the Germans pioneered the underground missile silo – building the first such complex near Saint-Omer in 1944. It was intended to store and launch V2s on England, but never entered service due to heavy Allied bombing. The British toyed with the idea of underground launch facilities in the 1950s for their missiles, and even built one, before opting for using submarines to launch their ICBMs. It was the Americans and Russians, however, who really took to the idea of planting nuclear missiles underground – as though that would help in the event of World War III. The primary purpose of the silos was to insure the rapid launch of Soviet UR-100 or American Titan II missiles to ensure mutual destruction. Since the Cold War, the Russians have moved most of their ICBMs to submarines, the Americans have about 450 silos still in operation, and China has built a few silos but still favors mobile ICBM launchers.