A mother ship, mothership or mother-ship is a large vehicle that leads, serves, or carries other smaller vehicles. A mother ship may be a maritime ship, aircraft, or spacecraft. Examples include bombers converted to carry experimental aircraft to altitudes where they can conduct their research (such as the B-52 carrying the X-15), or ships that carry small submarines to an area of ocean to be explored (such as the Atlantis II carrying the Alvin). A mother ship may also be used to recover smaller craft, or go its own way after releasing them. A smaller vessel serving or caring for larger craft is usually called a tender.