From wikipedia:

The Caspian Sea Monster, officially «KM» (Korabl Maket, Russian - Корабль-макет Naval Prototype),[1] also known as the "Kaspian Monster", was an experimental ekranoplan, developed at the design bureau of Rostislav Alexeyev.

The KM was designed in 1964 – 1965, and was unique in size and payload. The first spy photographs from American spy satellites showed a strange aircraft carrying letters "KM" on its fuselage. CIA disambiguated it as "Kaspian Monster", while it actually meant "Korabl maket" – "prototype ship" in Russian.

The ekranoplan had wingspan of 37.6 m, length – 92 m, maximum take-off weight – 544 tons. Until An-225 it was the largest aircraft in the world.

KM was designed as a special vehicle for the military and rescue teams. However designing such a machine caused serious difficulties. It was documented as a marine vessel and prior to the first flight a bottle of champagne was broken against its nose. It displayed the Soviet Navy Flag and was assigned to the Soviet Navy, since the ground effect is only possible within several meters from the surface. The new vehicle was, however, piloted by air force test pilots.