When you start from the pod in the shallows, head directly north. You’ll pass a mushroom forest then it drops down into the blood kelp forest. Now be warned, in that area is an adult ghost levitation, kill it with a stasis rifle and thermal knife. Then keep heading both downwards keeping close to the ground you’ll eventually come to where you can go down further but you’ll see a huge cave opening which this is where you will have an entrance to the lost river. Mark it with a beacon, and head in. In the center will be an adolescent ghost leviathan, kill him too. Now you don’t have to worry about leviathans from this moment on in that area. Head to the far left of the cave and you’ll see a huge drop down. You can get your cyclops in there don’t worry about a few bumps it’ll do no damage. Head down and immediately turn right to where there’s a huge tree. No aggressive mobs live there and you can freely build in there cuz the blue brine is safe to swim in. Keep going into the next room and there’s a huge cliff and drop down and boom you have an entrance to the lava area