The Great Flood refers to the most devastating flood and the most catastrophic geological event that has ever taken place in human history.
Virtually nothing historical, from writings to civilization to long-living organisms like trees, has survived from before about 3000 or 3300 B.C., when the Great Flood occurred. Today 43-60% of Americans accept as truth the Biblical account of the Great Flood, and many cultures record a world-destroying flood in their oral or written histories.
This flood was the means of God's judgment on mankind because of the evil in their heart and actions. God warned Noah that He was going to destroy the world with a flood, because of the wickedness of mankind. However, it is also noted that the flood was unleashed mainly to destroy the Nephilim whom were a cross between angel and Humans. The resulting union between the two created the Nephilim and plunged mankind into further depravity. Noah was to build an ark and take his family and pairs of each kind of air-breathing animal in it in order to be saved from the flood.
After Noah and his family and all the creatures were on board, God shut the door of the ark. The flood was brought on by the bursting open of the fountains of the great deep and the opening of the floodgates of the sky. The rain lasted nearly six weeks (40 days and nights). The flood waters apparently continued to rise for 150 days, but it was just over a year before the waters had receded enough for the occupants of the ark to leave.
The ark came to rest on one of the mountains of Ararat. God used a rainbow in the clouds as a sign that he would never again destroy the world in a flood.