World War II and the subsequent Cold War accelerated technological development especially for marine geophysics. Submarine warfare demanded better sonar, a technique like radar that uses reflection of sound waves to detect the shape of the seafloor and enemy vessels. The Navy also used magnetometers to detect the metal of submarines and ships and distinguish them from the background magnetism of the seafloor; they needed to map seafloor magnetism because this would have interfered with their submarine detection system. In the 1950s a careful naval survey of the Pacific North West coast of the US-produced a detailed magnetic map that was to prove important in understanding seafloor spreading. Much of this data was classified for some years after it was obtained.