Gen 6:5 And God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.
Verse 11,12,13 The earth also was corrupt before God, and the earth was filled with violence. And God looked upon the earth, and, behold, it was corrupt; for all flesh had corrupted his way upon the earth. And God said unto Noah, The end of all flesh is come before me: for the earth is filled with violence through them; and, behold, I will destroy them with the earth.
Gen 7:21,22 And all flesh died that moved upon the earth, both of fowl, and of cattle, and of beast, and of every creeping thing that creepeth upon the earth, and every man: All in whose nostrils was the breath of life, of all that was in the dry land, died.
Because of mans' sin, the earth was, in God's eyes, corrupt (spoiled, ruined). He wanted it cleansed and the two most potent cleaning agents in nature are water and fire. In this case, he chose water. When the flood was over, the earth was renewed (see First Heaven and Earth). Not only was sinful man gone, but also everything that had been tainted by him.
Gen 6:8,9 But Noah found grace in the eyes of the Lord. These are the generations of Noah: Noah was a just man and perfect in his generations, and Noah walked with God.
Verse 14 Make thee an ark of gopher wood; rooms shalt thou make in the ark, and shalt pitch it within and without with pitch.
Gen 6:17,18 Behold I, even I, do bring a flood of waters upon the earth, to destroy all flesh, wherein is the breath of life, from under heaven; and every thing that is in the earth shall die. But with thee will I establish my covenant; and thou shalt come into the ark, thou, and thy sons, and thy wife, and thy sons' wives with thee.
Verse 22 Thus did Noah; according to all that God commanded him, so did he.
Because of his obedience to God's ways, Noah was chosen to survive the wrath of God. He was given very specific instructions, and, because he obeyed, he and his family were carried through the flood to establish a new generation of men to replenish the earth. In Genesis 9:1-17 God placed a rainbow in the clouds as a symbol of his covenant with Noah and all his descendants that there would never again "be a flood to destroy the earth."
The Bible was presented to mankind by God as an explanation of his will and his plan for us. When learning a new subject we must start with the basics, a foundation upon which all later understanding is based. The first several books of the Bible are presented as an history, a chronicle, of God's plan from its' beginning. Speaking of the entire old testament; Romans 15:4 says: "For whatsoever things were written aforetime were written for our learning..." This point is reinforced in I Cor. 10:6, II Tim 2:16,17 and many other places. The details surrounding the Flood and throughout the book of Genesis are very specific, including genealogies from Adam to Noah and down through his sons to the birth of Israel and many other nations. Noah is referenced as a righteous man in II Pet 2:5 and Heb 11:7. Christ referred to "the days of Noe" in Matt 24:37 and Luke 17:26, comparing them to the time of his return.