A hovercraft (air-cushion vehicle, ACV) is a craft capable of traveling over land, water, mud or ice and other surfaces both at speed and when stationary. They operate by creating a cushion of high-pressure air between the hull of the vessel and the surface below. Typically this cushion is contained within a flexible "skirt". Hovercraft are hybrid vessels operated by a pilot as an aircraft rather than a captain as a marine vessel. They typically hover at heights between 200mm and 600mm above any surface and operate above 20 knots and can clear gradients up to 20 degrees. The first practical design for hovercraft derived from a British invention in the 1950s to 1960s. They are now used throughout the world as specialised transports in disaster relief, coastguard, military and survey applications as well as for sport or passenger service. Very large versions have been used to transport hundreds of people and vehicles across the English Channel whilst others have military applications used to transport tanks, soldiers and large equipment in hostile environments and terrain.