Genesis 7:10

"And after the seven days the floodwaters came on the earth."  Genesis 7:10

So after a week of bringing the animals into the ark, the flood starts.  Does this mean it started raining or does it mean water suddenly burst forth and covered the land?  I don't know. 

The commentary from Bible Ref notes that the Hebrew word for "earth" used in this verse is often used for a region instead of the entire planet, so there are some that say that the flood only destroyed the regions that were inhabited by mankind. 

The National Center for Science Education has an article arguing that the flood was only a local flood using scientific evidences derived from rocks and fossils left behind.  BioLogos agrees that it was only a local flood saying that we should interpret the flood story through the lens of the ancient Hebrews.  This article points out pretty early on that Christians have three options when scientific discoveries in God's creation seem to conflict with our interpretations of God's word.  We can abandon our faith in favor of the science, deny the scientific evidence to keep our interpretation, or reconsider our interpretations of scripture - the science might not actually disagree with the Bible, but rather our interpretations of it.  Evidence for God uses the scriptures to argue in favor of the local flood.

On the opposite side of the argument CBN has an article with scriptural evidence (including words from Jesus) that the flood was a global event.  Likewise, Creation Ministries International also argues for a universal flood stating that such a flood would account for the rocks and fossils that cover immense areas.  Don Stewart spells out a clear argument for the universal flood pointing out that about 1600 years before God told Adam and Eve to be fruitful and multiply so there could have been several hundred million people on the earth by the time Noah built the ark and they probably wouldn't have stayed confined to the Mesopotamian valley. 

The Amazing Bible Timeline with World History summarizes both sides into 7 short bullet points.  Both sides seem to have valid points and some convincing arguments.  Personally, I think I still lean towards the global flood, but whether the flood was a global event or just local event it is not the breaking point for my faith.  Either way, God told Noah he was going to wipe out humanity except for him and his family and he did.