"Did the Flood Only Cover the 'Known World' of Noah's Day?"

"
Regarding the Great Flood, whilst a literal physical fact,
it was also theologically necessary that the flood
covered the entire world, this typifying the teaching that
this earth will eventually be cleansed by fire after which
only God's true people will remain. The point is: God is
able to completely cleanse this world from all sin and
start afresh because He has done it before. To state that.."

http://www.ukapologetics.net/15/floodnotlocal.htm