During the Cold War, the Central Intelligence Agency (CIA) sent a few high altitude spy planes known as U-2's over the USSR to find out what they were developing as far as Intercontinental Ballistic missiles (ICBM's) go. One of these spy planes was shot down by Soviet anti-aircraft fire. The plane crashed deep in Soviet territory and as a result the pilot was sentenced to ten years in prison. The U.S. brought many excuses for the crash to the Soviets. One was that it had been a weather research plane that had accidentally strayed into Soviet airspace. Another was that the pilot had reported oxygen trouble and crashed due to that. However all this was proved wrong when the Soviets brought the pilot out (having only previously showed the U.S. the plane). This incident brought the U.S. relations with the Soviets farther down. Fortunately for the pilot, he was exchanged with a Soviet spy after only 1 and 2/3 years in prison.