Arecibo observing room. Astronomer Pedro Torres working in the observing room of the Arecibo radio telescope, Puerto Rico. At upper left is the 90- tonne dome which houses of the gregorian subreflector system. This radio telescope is situated in a natural crater in the mountains. The subreflector system has two mirrors and is suspended 130 metres above the fixed dish. To scan the sky the dome is steered above the 305 metre dish. Arecibo is used by the SETI (Search for Extraterrestrial Intelligence) institute amongst others. SETI is an organisation dedicated to investigating evidence for the existence of intelligent life elsewhere in the universe.