According to the story told in chs. 6–9 of the book of Genesis, God became so exasperated when he saw the corruption that had overcome the world he had created that he decided to destroy everything in it with a great flood. There was, however, one righteous man left – Noah. He was commanded by God to build an ark, into which he was to take his own family and representatives of all living animals. It rained for forty days and forty nights and the sinful world was drowned, but Noah and his companions survived in the ark. When God finally allowed the waters to recede, Noah began to send out birds. This was common practice among ancient navigators to see whether any dry land was near; if a bird did not return, it meant that it had found somewhere to land. After some delay, eventually Noah and his companions emerged from the ark and a new era for humankind began. God then made a covenant with them that he would never again destroy the earth, and he established a special sign as confirmation of this, a rainbow (a natural phenomenon which features in the mythologies of many religions). For Christian writers, the story of Noah's flood reveals key aspects of the relationship between God and the human race, especially the concepts of judgement (for humankind's sins) and salvation (for those, like Noah, who put their trust in God).