After commanding Noah to build an ark, God informs Noah in Genesis 6:17 exactly why he would need such a massive ark:
Behold, I, even I am bringing the flood of water upon the earth, to destroy all flesh in which is the breath of life, from under heaven; everything that is on the earth shall perish. (Genesis 6:17)The pre-flood world would be judged with a flood of water, but this flood was not a natural disaster. This flood was a supernatural disaster— a supernatural judgment. The coming flood was not a gift from mother nature. Mother nature does not reign over the weather in the universe, God reigns over the weather in the universe. He is the one who is in control.
God is the one who would send the flood. And this flood would not be of minuscule proportions. This flood would not merely glaze the local area. Instead, this flood would fill the whole world. It would destroy all flesh— everything under heaven. Everything on the earth would perish (not just the few unlucky individuals who happened to live near Noah).
Water is meant to be a source of nourishment. It is a gift from God enjoyed by both the righteous and wicked: He causes His sun to rise on the evil and the good, and sends rain on the righteous and the unrighteous. (Matthew 5:45). But because of sin, water which was created to nourish the body, can also be used to bring about the destruction of the body.
When the pre-flood world refused to drink the water of life, they received in return waters of death. When man refuses to drink from that water which would forever quench his thirst (John 4:14), then there is no hope. The pre-flood world repeatedly denied this water, and although God strived with them, they refused to embrace Him. Thus the flood was coming, and the ark over the next 120 years would stand as a condemnation of the world.
God's Covenant With Noah: Promised - Genesis 6:18
State Of The World (2469 BC): 120 Years Before The Flood - Genesis 6:3
God's Ark Which Noah Built - Genesis 6:14
The Pre-Flood World Soon To Be Blotted Out - Genesis 6:7
Noah Found Favor In The Eyes Of The LORD - Genesis 6:8