An espionage balloon is a balloon used for spying. Espionage balloons were a development of the observation balloons used even before the World War I behind the front line. Unlike observation balloons, espionage balloons were neither manned nor moored to the ground; they were designed to fly deep into hostile territories, where they would record intelligence data and transmit them back to the home base. As such, espionage balloons could only be operated successfully when the weather forecast and the wind direction were favorable. They were more economical (and indeed dispensable, often falling victim to air defense) than manned reconnaissance aircraft, as they did not put flying personnel into harm's way. They were usually hot air balloons.[citation needed]