Saturn Valley made appearances in EarthBound and Mother 3 as the village where the race of aliens known as Mr. Saturn lived. It was usually under attack of some sort, and in Mother 3.5 it is no different. It is situated on Twister Island. To reach it, journey to the volcanic area and climb to the highest point where there is an overlook of the largest magma crater. There should be a hole in the ground around here. Follow it downwards using the many ladders through the cave and at the other end of the tunnel you'll reach this elusive village. If you save the Mr. Saturns from the Pigmasks, you might learn something about the Third Cave of the Future.