In 1982, the first megamaser was discovered in the ultraluminous infrared galaxy Arp 220. The luminosity of the source, assuming it emits isotropically, is roughly . This luminosity is roughly one hundred million times stronger than the typical maser found in the Milky Way, and so the maser source in Arp 220 was called a megamaser. At this time, extragalactic water (H2O) masers were already known. In 1984, water maser emission was discovered in NGC 4258 and NGC 1068 that was of comparable strength to the hydroxyl maser in Arp 220, and are as such considered water megamasers.