Korus is the Lost Sea of Mars. In the days when water was plentiful, Korus filled the region that is now called the Valley Dor. After the seas and oceans disappeared, the Martian people began to believe that the valley was the road to Heaven.  Thus a common Martian tradition began, wherein the Martian people, upon reaching the age of 1,000 years, would make a pilgrimage down to the Valley Dor to seek out the world of the dead, where the oceans must surely have gone. This belief was very tragically inaccurate, for in the intervening years between the seas drying out and the beginning of the pilgrimages, the valley became inhabited by all manner of predatory beasts.