Why is there no mention of the Flood in the records of Egyptian or Mesopotamian civilizations which existed at the time?
Biblical dates (I Kings 6:1, Gal 3:17, various generation lengths given in Genesis) place the Flood 1300 years before Solomon began the first temple. We can construct reliable chronologies for near Eastern history, particularly for Egypt, from many kinds of records from the literate cultures in the near East. These records are independent of, but supported by, dating methods such as dendrochronology and carbon-14. The building of the first temple can be dated to 950 B.C. +/- some small delta, placing the Flood around 2250 B.C. Unfortunately, the Egyptians (among others) have written records dating well back before 2250 B.C. (the Great Pyramid, for example dates to the 26th century B.C., 300 years before the Biblical date for the Flood). No sign in Egyptian inscriptions of this global flood around 2250 B.C.
