Harry Turtledove & L. Sprague deCamp 
Down in the Bottomlands: And Other Places 

In the real world, the Mediterranean basin dried up several times, only to refill again. If it hadn’t, what’s really sea bottom would have become the most savage desert in the world. Mild-mannered Radnal vez Krobir likes it that way. When his kingdom’s enemies try to shake down the Barrier Mountains and let the Atlantic in, he has to stop them – or else drown along with everything he holds dear.