Radio astronomy began in 1931 when Karl Jansky discovered that the Milky Way was a source of radio emission while doing research on terrestrial static with a direction antenna. Building on Jansky's work, Grote Reber built a more sophisticated purpose-built radio telescope in 1937, with a dish; using this, he discovered various unexplained radio sources in the sky. Interest in radio astronomy grew after the Second World War when much larger dishes were built including: the Jodrell bank telescope (1957), the Green Bank Telescope (1962), and the Effelsberg telescope (1971). The huge Arecibo telescope (1963) was so large that it was fixed into a natural depression in the ground; the central antenna could be steered to allow the telescope to study objects up to twenty degrees from the zenith. However, not every radio telescope is of the dish type. For example, the Mills Cross Telescope (1954) was an early example of an array which used two perpendicular lines of antennae in length to survey the sky.