I once before asked a question on how to quickly submerge a large landmass/continent.
The answers there basically concluded that literally ''Sinking'' (a la Atlantis) is pretty much impossible to do fast.
So I have thought about it a lot more and came up with the following scenario: Imagine a large mass of land that is basically a huge valley surrounded on all sides by huge mountain ranges. The valley itself is below sea level (a lot below sea level) but the peaks of these surrounding mountains are above sea level and thus keep the enormous valley from flooding. Could I justify the quick flooding of this huge (think roughly Germany sized) valley by one of the mountains in this range by way of some freak (if need be magic) disaster suddenly becoming much lower therefore allowing the surrounding sea to flood in and flood the entire valley leaving only the peaks of the mountain ranges to stay above water?