Blox Fruits Underwater City Secrets Guide: All 3 Stars

Underwater City hides three Island Secrets, the objective type added to First Sea islands in Update 30. None of them are found by wandering. Each one only exists after you accept it from a secret Quest Giver NPC standing inside the domed city, and each one ends with you walking back to that same NPC to collect a star on your world map.

Quick answer: Accept each secret from the Quest Giver in Underwater City, then complete them in order — ride a rising bubble into the hidden cave and kill the cursed guardians, aim the reflected light beam at the wall target to open the Water Kung-fu Teacher’s room, and open clams during the Underwater City boss event to awaken and defeat the Fishman Lord. Return to the Quest Giver after each to claim the star.

Before you start: access and level requirements

Underwater City sits in the First Sea and is built for levels 375 to 450. The entrance is the whirlpool ringed by three small rocky islands, located between Frozen Village and the Prison. Sail or fly into the center of the whirlpool and you get pulled down into the dome. If you cannot spot the islands, raise your graphics quality — the render distance is what usually hides them.

Once inside, open your world map and look for the star icons attached to the island. Those three empty stars are the secrets, and they fill in one at a time. The Quest Giver who hands them out is separate from the standard King Neptune grinding quests, so talking to the regular quest NPC does nothing for this. Blox Fruits also gates the secrets behind acceptance, meaning the bubble, the puzzle rocks, and the clams will not respond until the matching quest is active.

Secret	Trigger	Where
Bubble cave	Ride a rising bubble, open the chest, defeat the cursed guardians	Floating bubbles around the city
Light puzzle	Align the reflective rocks so the beam hits the wall target	Right side of the main boss castle
Awakened boss	Open clams during the Underwater City boss event, then kill Fishman Lord	Across the island, event only

Secret 1: Ride the bubble to the hidden cave

Talk to the Quest Giver and accept the first secret. The objective updates immediately, and the bubbles around the city become interactive.

Look for a rising bubble drifting through the city. They spawn low to the ground and float upward, so watch the open space between buildings rather than the rooftops.

Step inside the bubble and stop moving. It carries you upward and a message confirms the bubble is taking you to a hidden cave.

Player floating toward a golden marker as an on-screen message says a bubble is carrying them to a hidden cove — The bubble lifts you out of the city and drops you at the hidden cave marker. Image: Gamer Robot Inc

Inside the cave, open the chest. This spawns the cursed guardians, so be ready to fight the moment the chest animation finishes.

Clear every guardian. You will see confirmation that the cursed guardians have been defeated, followed by a prompt telling you to claim your reward from the quest giver.

Head back down to the Quest Giver and hand the objective in. The first star fills in on the map.

Secret 2: Solve the light reflection puzzle

Accept the second secret from the Quest Giver before going anywhere. The reflective rocks will not rotate until the quest is live.

Move to the right side of the main boss castle in the middle of the city. A movement ability makes this faster, but the puzzle area is on ground level and reachable on foot.

Ability panel with Portal Dash highlighted as the character dashes across the map — A dash move such as Portal Dash cuts down the travel time between the Quest Giver and the puzzle area. Image: Gamer Robot Inc

Interact with each reflective rock to rotate it. A yellow beam travels from rock to rock, so line them up one at a time and follow where the beam lands after every turn.

Point the final rock so the beam strikes the target marked on the wall. When it connects, the wall opens up and reveals a room.

Walk in and speak to the Water Kung-fu Teacher inside. He also sells the Water Kung Fu fighting style for $750,000 if you want it while you are there.

Water Kung-fu Teacher dialogue box offering training inside the opened room — The Water Kung-fu Teacher sits behind the wall the light beam opens. Image: Gamer Robot Inc

Return to the Quest Giver to bank the second star.

Secret 3: Awaken and beat the Fishman Lord

The last secret is the only one you cannot finish on demand. It requires the Underwater City special boss event to be running on your server, and the clams stay inert outside of that window.

Accept the third secret from the Quest Giver so the objective is already active when the event starts.

Travel to Middle Town and speak to the Secrets Master NPC. During the last 10 minutes of an hour, this NPC tells you which boss stirs next, so check whether Underwater City is the one queued up.

When the event goes live, return to Underwater City through the whirlpool.

Search the island for clams and open every one you find. They are scattered widely, so a fast movement option saves a lot of running.

Character standing in Underwater City with the ability panel listing movement moves open on screen — Sweeping the island for clams is much quicker with a mobility move slotted. Image: Gamer Robot Inc

Keep opening clams until the awakening cutscene plays. That cutscene is the confirmation the boss has been triggered.

Defeat the awakened Fishman Lord, the level 425 boss version of the island’s usual castle boss.

Fishman Lord level 425 boss health bar shown as the player closes in for the fight — The awakened Fishman Lord fight, listed at Lv. 425. Image: Gamer Robot Inc

Claim the final reward from the Quest Giver to light up the third star.

How to check all three stars registered

Open the world map and look at Underwater City. Three filled stars next to the island means every secret is logged. If a star is still empty after you finished the task, you almost certainly skipped the hand-in — completing the action alone does not count.

The most common reasons a secret refuses to progress:

The quest was never accepted from the Quest Giver, so the bubble, rocks, or clams do nothing.
The light beam is bouncing between rocks but never reaches the wall target, which leaves the wall sealed.
The Underwater City boss event is not running, so no number of clams will start the awakening cutscene.
You cleared the objective but did not walk back to the Quest Giver to claim the star.

Two of the three can be knocked out in a single visit, which makes the timing on the boss event the only real bottleneck. If Underwater City is not the island queued up, finish the bubble and the puzzle first, then keep an eye on the Secrets Master until the event rotates around.