First held in 1971, RIMPAC is a bi-annual naval exercise hosted by the US Navy. Short for Rim of the Pacific, RIMPAC is mainly attended by participants in that Ocean, but is also open to navies from elsewhere. Besides the active participants who send ships and aircraft, many countries send observers as well. Besides the US, Australia and Canada are the only countries to have participated in every RIMPAC to date. The exercises consist of maneuvers, drills, and even live-fire exercises. The primary objective is to improve cooperation between the worlds navies, to ensure the security of the sea lanes around the world.