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.

The largest in the world is the Arecibo dish in Puerto Rico, with a diameter of 300 meters (1,000 feet).

The project is called SERENDIP 4. Typically, the Arecibo dish scans all of the sky visible to it about once every six months, providing a thorough search for any ET.

Although SERENDIP has been in operation since 1976, the SERENDIP 4 sky survey will open a whole new range of possibilities when it searches for alien signals at what radio astronomers have nicknamed the "water hole".

This is a naturally quiet part of the radio spectrum which scientists speculate might attract interstellar communication.

Life as we know it

This relatively quiet frequency range contains the natural radiations of two common elements - hydrogen (H) and hydroxyl (OH). When these two elements are combined on Earth they form H2O - water - hence the region's nickname.

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.

"Water holes have been the gathering places for animals of all species throughout Earth's history," says Louis Friedman, executive director of the Planetary Society, which is helping to fund the search.