The direct path down to the goal is blocked by beams, so you have to take the long way around. Start by heading right from the Tyne Wellspring.
Snake your way through the hallways with red beams until you come to another Tyne Wellspring.
Keep heading down and right. A gate will close behind you and you'll face several waves of enemies.
There's a sonic mechanism with a 100 coin gem in it if you feel like you haven't fought enough yet.
The passage out is at the bottom right of the room. At the bottom of the wall on your right is a hidden alcove with a silver treasure in it.
Head through the lantern jellies on your right and down.
Turn left at the bottom and fight your way through the waves of enemies that spawn. Every room you pass into from here on will lock behind you and spawn enemies. Since you haven't seen a Tyne Wellspring in a while, you have to be careful about your health. Try to focus on the anglerfish first since they can do a lot of damage and are harder to avoid than the nettles or urchins. Don't forget to use your torpedoes as well.
Head down, then pass into the next room on the right and fight the enemies there.
Head into the next room on the right. There are three red crabs in here along with a ton of jellies. Skirt around the outside to try to only get the attention of one crab at a time. If the jellies surround you, use your sonar blast to drive them away.
Head up and out of this room to find a chest with a Tyne Collector inside.
Hit the lever and backtrack to the lantern jellies. Head through the gate on the left and then up to the Tyne Wellspring.
Pull open the gate below you and pass through it.
Keep heading left and pass through the red beams. This will bring you out below the solid yellow beams blocking your progress down.
A little below you, you'll find a Tyne Vortex. Just to the left, there's a passage that Merryn can take to find a red gem. Dodge the snaking yellow beams on your way in, then scare the turtle at the end for an easy escape.
Return to the sub and head down. You'll come to a hermit crab. Upgrade anything you can afford to. We recommend focusing on anything that gets you better drops like the Heat Repair for 500 coins or better healing like the Repair Bay for 1,200.
Go past the crab and keep heading down. Again, your path to your goal is blocked. Destroy the glass wall on your left and head inside the tunnel to the Tyne Wellspring.
Head to the left. The door will lock behind you and you'll face three Formori Sentinels. You face them one at a time, so just use the same tactics as when you faced them before.
Take the bottom right passage out of the room and follow it until it brings you to the purple turtle. Scare it to turn off the purple beams.
Head back to the main room and head to the top right. There is a sonic mechanism that will release a wave of enemies and a gold treasure.
Head to the top left hallway next. Immediately up that hallway is a hidden alcove and another gold treasure.
Keep heading up and snake your way through the red beams. Halfway through the curve, there is a tunnel for Merryn. Hop out of the sub and head down. The sub won't take damage while you're not in it. There's a chest at the bottom of the room below you, but watch for the jellies that spawn in there with you. Open the chest to claim your treasure worth 300 coins.
Head back to the sub and continue right and down around the curve. Hit the red lever to open the gate. This brings you back above the yellow beams, directly across from the first Tyne Wellspring.
Hit the wellspring, then come back to the main hall and head straight down. The yellow beams are off now, so you can make it all the way back down to the glass gate you destroyed on your left.
Go back into the round room where you fought the Sentinels and exit through the bottom left tunnel.
Use the alcoves to hide from the Red Reaper. There's a chest at the end with a treasure worth 500 coins. Wait for the Reaper to pass and pull open the chest. Press up against the wall on the right to avoid catching the Reaper's attention.
Head back to the main room and exit to the right. Head up and to the right to just below the yellow beams you turned off. Take the tunnel on the right and work your way across the red beam and down.
Follow the tunnel around to a Tyne Wellspring.
Work your way through the green beams in the next two hallways to another Tyne Wellspring.
Keep snaking past the beams and down until you see a red lever. Hit it to open the gate behind you.
Head back along the direction of the beams to find a room with the green turtle. Scare it and then head back along the hallway to the right.
Just before the lever, ping your sonar to reveal a path up to a chest and a 200 coin gem.
Make your way all the way back to the main tunnel down. Don't miss the gold treasure on your way.
Go down past the Tyne Vortex again and the hermit crab. Be sure to stop and spend any coins you have because this is your last chance before the final boss.
Keep descending until you enter Rimorosa's Lair.