The KM (Korabl Maket, literally ""Model-Ship""), known colloquially as the Caspian Sea Monster, was an experimental ground effect vehicle developed in the 1960s. The KM was the largest and heaviest aircraft in the world from 1966 to 1988, and its surprise discovery by the United States and the subsequent attempts to determine its purpose became a distinctive event of espionage during the Cold War. It was tested on the Caspian Sea for 15 years until 1980, when it was destroyed following a crash caused by pilot error. There were no human casualties, but the KM was damaged and no attempts were made to save it, it being left to float before eventually sinking a week later. The KM was deemed too heavy to recover and has remained underwater at the crash site ever since. The KM remained the largest aircraft in the world during the entirety of its existence and is the second-largest aircraft ever built, behind the Antonov An-225 Mriya that flew for the first time in 1988.