Airships are lighter-than-air vehicles that are lifted by gasses less dense than the surrounding air, usually helium. They offer flight capabilities at a fraction of the fuel use, and overall carbon footprint, of airplanes. At their most popular before the second world war, airships fell out of use due to the rise of the airplane industry and a series of high profile accidents in the mid-20th century. Now, new models of airships, with improved materials (including non-flammable gasses), electronic instruments and aerodynamics, is being developed. Some foresee passenger airships in the not-too-distant future.