Scripture is clear about the historic reality of a global Flood in Noah's day. Genesis 7:19–22 specifically says that "all the high hills under the whole heaven were covered" as "the waters prevailed 15 cubits [26 feet, or 8 m] upward." All air-breathing land animals and people that were outside the Ark also died (Genesis 7:22–23).
Today, many people unfortunately do not accept the biblical account of a worldwide flood because they have been taught that most rocks and fossils were deposited over millions of years (and therefore not by a global Flood). Until the 1800s, most westerners believed what the Bible records about the earth's recent creation and the global Flood. The secular idea of millions of years did not gain extensive popularity until the 1830s, under the influence of a man named Charles Lyell.
Based on how slowly many rock layers seem to form today, Lyell rejected the Bible's claims and declared that the earth's many rock layers must have been laid down slowly over millions of years. But he never witnessed the actual formation of the earlier rocks to see whether they could be laid by a unique, one-time global Flood unlike anything we observe today. Lyell's claim was based on his own preconceptions, not his observations.
[Read the rest of the article at Answers in Genesis.]