Lost In Deep Seas is an Archon quest under Chapter 4 Act 3 of Genshin Impact.
Your objective is to look into the hidden rules and find the pipes where Childe disappeared to.
Here’s a walkthrough and guide of Lost In Deep Seas in Genshin Impact.
Continue asking around for clues
After the quest begins, talk to Vidoc for clues. He’ll tell you about a hidden rule that there are cannibals on pipe-cleaning nights.
Ask the trio
Then, talk to the trio who hangs out together often as they may be related to the cannibals.
Ask the guard at the dormitories
Talk to Fielding outside the dormitories about the strange sound during pipe-cleaning night.
Return to the dormitories and rest after that.
The Traveler will then see a dream about Childe and the trio from earlier as they know each other. He then disappears into the pipes and something was calling for him.
Go to where the rumored pipes are
You will now need to sneak past a few guards to reach the rumored pipes.
You need to do this part according to the script or it may get bugged. I found the pipes but the story didn’t load as I didn’t follow the path that the game wanted me to go.
First of all, you’ll see a guard on the walkway. Go to the inner circle on the left to avoid that guard.
The next guard will walk away and look the other way.
Go past that guard and you’ll see a lady guard in the distance. Head to the outer circle on the right to avoid the lady.
You will then see the pipe where there’s greenery in it. Go inside.
You’ll meet the trio inside.
Enter the pipe
Continue inside the pipe to trigger the cutscene.
Return to the dormitories after as you’ll need to find a diver.
The quest ends here. Act 3 also ends here.