The water quadrant of Bubble 543 is the ocean surrounding the central island on the Fifth Floor of the dungeon. Its castle objective is the bridge of the Akula, a huge submarine beneath the surface. Capturing that bridge liberates the quadrant, but the submarine's machinery continues to move water into the island's submerged interior. The sea consequently remains an active part of the bubble's other castle campaigns. Its sharks, jellyfish, currents, and extreme depth also make a later return to the Akula considerably more dangerous than the initial capture suggests.
The ocean and the Akula
Bubble 543 divides its castle objectives among environmental quadrants. The water quadrant forms a ring around an island occupied by the land and upper regions of the bubble. The Akula lies underwater, while passages associated with the submarine connect the sea to the island's interior. The arrangement makes the quadrant's objective more than a separate naval encounter: the condition of the vessel and its pumps affects people trying to clear the other areas.
The submarine belongs to the story of Chaindrive, the bugbear who sought the Gate of the Feral Gods in the Necropolis of Anser. He used his vessel to find an underwater entrance near the tomb and released the mechanical bird Ruckus to interfere with other seekers. By the time the crawlers arrive, the Akula is an aging, dangerous structure rather than an orderly ship under its captain's control. Its old defenses can nevertheless be reactivated. While Carl confronts Ruckus elsewhere in the bubble, the bird's description changes to show that it has received new orders from the submarine.
Chris Andrews and other crawlers assigned to the water quadrant enter the Akula before Carl visits it. They encounter robots inside and awaken a bugbear head preserved in a jar and fitted with mechanical spider legs. Chris later tells Carl that he broke the jar and that the group took the bridge. The bubble then announces the bridge's occupation, names Chris as the crawler who took the throne room, and declares the water quadrant liberated. Its originating crawlers gain permission to travel to the other quadrants. The conquest removes the castle requirement for this part of the bubble; it does not empty the ocean or neutralize its inhabitants.
The change in control has an immediate physical consequence. The Akula fires a torpedo into the mountain after the bridge is taken. Water begins rushing through the resulting opening, and the damaged submarine starts to flood. Chris describes three escape tubes, each leading toward a different quadrant. He reaches Carl's area, while two other survivors of the water party, Vadim Zbar and Britney Proskurina, reach the land quadrant. Chris's account explains both his sudden arrival outside the sea and the water pouring into the island. It also leaves a practical problem: the submarine's pump is still running.
Water in the neighboring quadrants
The pump sends seawater into the Necropolis of Anser, the subterranean quadrant beneath the island. People there retreat to saferooms near the top as tunnels fill. Water-breathing scrolls allow limited exploration, but they do not remove the tomb's other hazards. One crawler, Mike, is killed by a trap in a flooded passage; his companion Bobby retreats. Clearing the water becomes a condition for making a sustained attempt on the necropolis, even though the water quadrant's own castle has already fallen.
Flooding also changes the attack on the land quadrant's sandcastle. Gwen's group finds a drainage control beside the necropolis wall. Opening it would send water back toward the surrounding sea through a tunnel and wash away the sandcastle built around that outlet. Gwen favors this direct route. Katia argues for entering the castle because its mage holds a winding box the group needs. The two approaches therefore put a different value on the same body of water: it can destroy the objective quickly, but a flood may also eliminate the chance to recover an item from inside.
Carl recognizes that the order of the bubble's castle conquests has shaped these choices. Taking the water quadrant first fills the subterranean area; that stored water then supplies a way to break the land castle. This is not merely a matter of swimming through adjacent zones. The Akula, the mountain opening, the necropolis, and the drainage outlet create a connected route through which actions in one quadrant alter the obstacles in another. The group's decision to seek the winding box delays use of the simplest drainage plan and keeps the pump problem unresolved.
During the sandcastle operation, the drainage valve is opened only partway, but the released water is powerful enough to destroy the castle. Carl and Donut are caught in the discharge and hurled into the sea. Both are hurt, and Donut briefly loses consciousness. Once they can orient themselves underwater, they must still reach the surface and escape the marine predators drawn to the disturbance. The sandcastle's destruction completes the land objective, yet it leaves its stairwell buried near the outlet and exposed to continuing spray. Gwen's group begins digging the chamber out and arranging to shield it from the water.
The emergence into the ocean reveals how little the earlier victory has made it safe. Carl sees black, red-eyed concierge sharks converge on an injured shark. Their fight develops into a feeding frenzy, giving him an opening to swim away with Donut. He uses a kayak carried in his inventory to bring her back to shore. From the surface he can see the water blasting out of the necropolis wall and the remains of the sandcastle. The Akula is on the far side of the ring-shaped quadrant, too far away for an improvised swim. Turning off its pump will require a separate expedition.
The return dive
Britney and Vadim give Carl's group firsthand knowledge of the sea. They had survived the original assault on the Akula with Chris, but Britney refuses to dive again and warns that those who return will die. Vadim agrees to guide the mission and believes he can find the pump controls. Carl, Katia, Tran, and Vadim plan to enter the vessel and stop the flow while Gwen's group manages the drain on land. The plan treats the submarine as a working part of the bubble's hydraulic system, rather than as a completed castle to be ignored.
Reaching the ship involves more than avoiding the sharks seen from shore. Vadim describes jellyfish and other creatures at depth. Carl's team prepares a diversion to draw concierge sharks away from its approach. Katia then reshapes her body into a heavy diving bell, carrying Carl, Tran, and Vadim below the surface while Donut remains behind. The Akula is approximately 500 meters down. Water-breathing magic lets the crawlers survive the descent, but it cannot make the route predictable or guarantee that every part of the hull remains intact.
At close range the vessel is enormous, with its upper structure torn open and a strong current beneath it. Large blue jellyfish hang near the entrance. Katia spreads herself over the hull to watch for threats while the others move through the flooded interior. Vadim guides them past debris, old bugbear remains, and signs of the earlier fight with the ship's robots. The scene shows that the bridge's capture was a narrow success: much of the Akula still contains wreckage, monsters, and unexplored machinery. Carl takes a map from a corpse, gaining a clearer picture of flooded compartments and moving enemies.
The mission is interrupted before the team reaches the controls. Quetzalcoatlus, the ghost associated with the subterranean quadrant, appears inside the vessel and attacks. She has crossed a boundary that the crawlers might otherwise assume separates one castle's threat from another. The team then discovers dead changeling principals on the bridge, including Henrik. Their presence shows that another party has also used the submarine while pursuing its own aims in the necropolis. The ship moves violently, is torn from its position, and sinks much deeper. Carl abandons the immediate pump objective as survival and escape become urgent.
Lusca and the deepest water
The creature that seizes the Akula is Lusca, an immense octo-shark and the water quadrant's city boss. She carries a brood of juvenile octo-sharks in her mouth. The submarine is snapped apart and held there as food for them, trapping Carl's party inside the wreck. A staged announcement gives the crawlers a short interval before Lusca swallows what remains. Their safe compartment may resist the initial attack, but remaining there would leave them inside the creature and far below the surface.
Carl uses a bloodlust potion to provoke the juveniles into attacking one another. The frenzy greatly reduces their number, allowing the group to leave the sealed room and make for the open water within Lusca's mouth. The danger does not end when the smaller predators die. Lusca is still moving around the circumference of the quadrant, and her size and depth make a conventional escape impractical. The party must time its movement against her swallowing motion and the shifting remains of the Akula.
Carl casts Protective Shell as Lusca swallows. The spell's rigid protective sphere badly damages her upper mouth when the moving creature strikes it. Katia has also brought the enormous buzzsaw recovered from Ruckus, attaching it for use during their attempted escape. Their movement through the injured shark does not follow the intended route: they spin out into the sea and cut back through her head. The buzzsaw reaches Lusca's brain, killing her. This victory removes the largest known predator in the quadrant, but strands the crawlers near the ocean floor under damaging pressure.
The water is now itself a lethal obstacle. Carl's health drops rapidly at that depth, and a mass of jellyfish lies above the party. Vadim uses one of the Akula's escape tubes and rises toward the necropolis. Carl, Katia, and Tran instead use the Gate of the Feral Gods to leave the deep water. The pressure of the water forced through their briefly opened portal is enough to topple a metal wall near Pandinus on the land quadrant. The escape confirms that the same water which can erase a sandcastle can remain dangerous when moved through a much smaller opening.
After the castles
The sea continues to affect Bubble 543 after Lusca's death. Water is deliberately retained in the necropolis while Carl's group arranges to use lightning against Quetzalcoatlus. They connect the drainage equipment to a lightning tower and keep part of the interior flooded so the strike can carry through it. The plan depends on the earlier flow from the water quadrant, but it also threatens to conduct electricity into the sea. The group takes shelter rather than assuming that the water quadrant's liberated status makes it harmless.
When the bubble's upper barrier eventually opens, the water and island remain in place. From the edge, Carl can see that the lower bowl of the bubble still rises above the sea, with the dark lacuna beyond it. The quadrant therefore remains a distinct body of water even after its castle conditions are satisfied. Near the floor's end, Katia and Tran return to hunt its remaining sharks for experience. The ocean has ceased to block the bubble's castle progression, but it continues to contain mobs and to offer both danger and resources to the survivors.