In the mid 1900s it was discovered that many objects in the Universe shine in radio waves. This led to the development of radio telescopes, which are special antennas for detecting and imaging the radio signals from astronomical objects. Radio telescopes have yielded new insights into many aspects of the Universe, including black holes and cool gas. The largest single-dish radio telescope is the Arecibo Telescope in Puerto Rico. With a radio interferometer astronomers create very sharp images by combining the signals from many individual radio dishes spread over a large area. The Very Large Array (VLA) in New Mexico is one of the largest interferometers. It is sometimes linked with other arrays of telescopes outside the US, or even in space, to mimic an even larger configuration.