A blimp is a type of airship, also known as a non-rigid airship, that does not have an internal structural framework or keel. Instead, blimps rely on the pressure of the lifting gas inside the envelope and the strength of the envelope itself to maintain their shape. Blimps are filled with a gas that is lighter than air, such as helium, which provides lift and allows them to float. They can be steered and powered, and can move forward through the air under their own power like airplanes. Blimps are commonly used for advertising, freight transportation, tourism, camera platforms for sporting events, geological surveys, aerial observation, interdiction platforms, and some research purposes. They were also used by navies during World War I in convoy and antisubmarine patrol duty. Blimps are the most commonly built airships because they are relatively easy to build and easy to transport once deflated. However, their unstable hull limits their size, and a blimp with too long a hull may kink in the middle when the overpressure is insufficient or when maneuvered too fast.