Sending electromagnetic signals through the air became possible in the later 19th century with wireless telegraphy. The fact that signals were affected by natural interference was soon discovered. By the early 1900s, wireless telegraphy could cross the Atlantic. Intercepting the enemy's radio communications and interfering with their transmission was important in World War II. Messages in code were intercepted and recorded for decryption and translation. Not only can radio signals be detected, but those listening can be detected, since radio receivers also emit a signal.