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The planet Mars has interested scientists for hundreds of years. 1_______. This sounds surprising, but astronomers say that there could have been lakes, rivers and seas on the surface of the planet. 2_______. In 1877, the Italian astronomer Schiaparelli was looking through his telescope at the planet, when he noticed that there seemed to be a network of canals on the planet. 3_______. When the Americans sent Mariner 4 into space, it sent back 21 photographs, which show that Mars is a cold, lifeless world. Therefore, we can conclude that people could not be living there now, although they might have done so in the past. In 1976, the photographs taken by Viking 1 were examined, and one of them showed a strange pyramid-shaped rock which could be taken for a human face. Was this a trick of the light or evidence that a great civilisation used to exist on the planet? 4_______.