User blog comment:Kogath/Monster Hunter 4 Ultimate for the Nintendo 3DS/@comment-25192100-20141103061352/@comment-6050823-20141103225701

To add more to what Pike said, some of the Old Jungle's rivers connect to the underground lakes found in the Dede Desert in the Old World which is how Plesioth is able to go to the Old Desert. The Plesioths are able to go to the Sekumea Desert(2nd Gen. Desert) by both the sea and the underground lakes as well.