A military base is a facility directly owned and operated by or for the military or one of its branches that shelters military equipment and personnel, and facilitates training and operations. A military base always provides accommodations for one or more units, but it may also be used as a command center, training ground or proving ground. In most cases, military bases rely on outside help to operate. However, certain complex bases are able to endure on their own for long periods because they are able to provide food, water, and other necessities for their inhabitants while under siege. Bases for military aviation are called military air bases, or simply "air bases". Bases for military ships are called naval bases.