Dubbed "an ear to heaven," Observatorio de Arecibo contains the world's largest and most sensitive radar/radio-telescope. The telescope features a 20-acre (8-hectare) dish, or radio mirror, set in an ancient sinkhole. It's 1,000 feet (300m) in diameter and 167 feet (50m) deep, and it allows scientists to monitor natural radio emissions from distant galaxies, pulsars, and quasars, and to examine