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An ekranoplan (Russian name 'зкраноплан', translation: sea skimmer) is a ground effect vehicle resembling an aircraft. They operate on the principle of wing in ground effect, where the air gap between a wing and the ground is small enough for the air to be compressed. This effect can be felt when landing in a commercial air liner; just before touchdown, the angle of descent can be felt to decrease. Ground effect vehicles skim just above a flat surface, with the height being dependent upon the size of the vehicle.

During the Cold War, it has been sighted for years on the Caspian Sea as a huge, fast-moving object. The name Caspian Sea Monster was given by confused U.S. intelligence operatives, who were puzzled by 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" turned out to be a secret airplane of the Russian military, designed to fly only a few meters above water, thus saving energy and staying below enemy radar.

The KM, as the model was known in the top secret Soviet military development program, was over 100 meters long, weighed 540 tons fully loaded, and could travel over 400 km/hr mere meters above the surface of the water. Once moving at speed, the ekranoplan was no longer in contact with the water, and could move over ice, snow, or level land with equal ease.

The important design principle is that the wing lift reduces the further above the surface (land or sea) the ekranoplan "flies" (see ground effect). Thus it is dynamically stable in the vertical dimension.

These craft were originally developed by the Soviet Union as very high-speed (several hundred km/hour) military transports, and were mostly based on the shores of the Caspian Sea and Black Sea. The largest could transport over 100 tonnes of cargo.

Since the collapse of the Soviet Union, commercial development has occurred mostly in the United States.

The term ground effect vehicle is often misapplied to hovercrafts, which use fans to form an air cushion (hence the correct term air cushion vehicle) rather than relying on true ground effect.

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