An array of antennas at the centre of the primary dish of the Arecibo Observatory in Puerto Rico. A gap in the panels that comprise the main reflector of this radio telescope can be seen at centre, showing that the reflector is suspended above the local vegetation in a natural hollow in the limestone scenery. Apart from being the world's largest radio telescope for many years (305m in diameter), Arecibo is also known for its use in the search for extraterrestrial intelligence (SETI).