A hovercraft is a vehicle which moves on top an air cushion, air being forced under the vehicle by its engines and held in place by the curtain. The theoretical grounds for motion over an air layer were constructed by Konstantin Eduardovich Tsiolkovskii in 1926 and 1927. The first design that would be recognized as a true hovercraft (Pintaliitäjä) was designed by Finnish aero engineer Toivo J. Kaario in 1931. Kaario's efforts were followed closely by Vladimir Levkov in the Soviet Union; his L-5 fast-attack boat reached 70 knots (130 km/h) in testing. The birth of modern hovercraft is associated with Sir Christopher Cokerell, who constructed his SR.N1 in 1959. It was not until a few years later when the military has shown interest in the new vehicle, which could move over land or water with great speeds. The biggest developer of hovercraft was Soviet Union, including the world's largest Zubr class with cargo capacity in excess of 100 tonnes. By the end of the 20th century, many navies used limited number of hovercraft as landing ships or fast attack craft.