What is an Ekranoplan

An ekranoplan (Russian: экранопла́н, literally "screen plane") is a vehicle resembling an aircraft but which operates solely on the principle of ground effect. Ground effect vehicles (GEV) fly above any flat surface, with the height above ground dependent upon the size of the vehicle. Ekranoplan design was conceived by Soviet engineer Rostislav Alexeev.

During the Cold War, ekranoplans were sighted for years on the Caspian Sea as huge, fast-moving objects. The name Caspian Sea Monster was given by US intelligence operatives who had spotted the huge vehicle, which looked like an airplane with the outer halves of the wings removed. After the end of the Cold War, the "monster" was revealed to be one of several Soviet military designs meant to fly only a few meters above water, saving energy and staying below enemy radar.

The ekranoplan has a lifting power of 1,000 tonnes, among the largest ever achieved. The KM, as the Caspian Sea Monster was known in the top secret Soviet military development program, was over 100 m long (330 ft), weighed 540 tonnes fully loaded, and could travel over 400 km/h (250 mi/h), mere meters above the surface of the water.

(The video is documentary in Russian)

Source: Wikipedia