The Thrashing Sea is a body of water separating the Illud Highlands from the mainland of the Middle Kingdom. It was once kept back by an Yttronium barrier created by the Ytarrans (who had their capital on what are now the Highlands), but eventually these barriers failed and the Ytarrans were forced to reloacte their capital to what is not Tarquazhan.
Notably, this blocking of the Thrashing Sea was the last straw that drove the Cleansers to begin their murderous rampage against the Ytarran Empire.