The Arecibo dish in Puerto Rico is 300m across The most sensitive search for ET has begun, using the world's largest radio telescope. Our Science Editor David Whitehouse reports.
Astronomers believe that radio telescopes offer the best way to find intelligent life in space.
For nearly 40 years scientists have used them to try to detect signals from any extra-terrestrial beings that may be out there. This is a naturally quiet part of the radio spectrum which scientists speculate might attract interstellar communication. Since water is an essential element of life as we know it, extra-terrestrials might well select that frequency to carry signals to other worlds. The water hole of the radio spectrum might be the place where far-flung alien species communicate across the vast reaches of interstellar space. The BBC is not responsible for the content of external internet sites.