Carriers have evolved since their inception in the early twentieth century from wooden vessels used to deploy balloons to nuclear-powered warships that carry dozens of aircraft, including fighter jets and helicopters. As of 3 March 2016, there are thirty-seven active aircraft carriers in the world within twelve navies. The United States Navy has 10 large nuclear-powered carriers (known as supercarriers, carrying up to 90 aircraft each), the largest carriers in the world; the total deckspace is over twice that of all other nations' combined. As well as the supercarrier fleet, the US Navy has nine amphibious assault ships used primarily for helicopters (sometimes called helicopter carriers); these can also carry up to 25 fighter jets, and in some cases, are as large as some other nations' fixed-wing carriers.