Fu-Go was an incendiary balloon weapon deployed by Japan against the United States during World War II. It consisted of a hydrogen-filled paper balloon 33 feet (10 m) in diameter, with a payload of four 11-pound (5.0 kg) incendiary devices and one 33-pound (15 kg) high-explosive anti-personnel bomb. The uncontrolled balloons were carried over the Pacific Ocean from Japan to North America by fast, high-altitude air currents, today known as the jet stream, and used a sophisticated sandbag ballast system to maintain their altitude. The bombs were intended to ignite large-scale forest fires and spread panic.