Communication with submarines is difficult because radio waves do not travel well through good electrical conductors like salt water. The obvious solution is to surface and raise an antenna above the water, then use ordinary radio transmissions. However, a submarine is most vulnerable when on the surface. Early submarines had to surface frequently for oxygen needed by their diesel engines to charge their batteries. During the Cold War, however, nuclear-powered submarines were developed that could stay submerged for months. To communicate with submerged submarines several techniques are used.