Ocean world

A planet only described as an ocean world appears in chapter 6 of Metroid EX. It has oceans of concentrated sulfuric acid, and a human population.

Samus Aran, Joey Apronika and Diesel come to this planet to investigate suspicious events. Their suspicions are confirmed when Joey and Diesel, in their excitement, nearly jump into the acidic ocean. In town, Samus learns that the acidity of the water is the result of a "Sea Demon" falling into it. Returning to the beach, Samus is confronted by Defen, the final boss of the Deadly Six Stars, who she assumes to be the sea demon. He throws Joey and Diesel into the acid, provoking Samus into diving in.

He attacks Samus underwater using Rastodon, the true sea demon, and severely damages her Power Suit. Falling to the ocean floor, Samus triggers a dormant volcano into erupting, which kills Defen and Rastodon. With Rastodon's death, the townsfolk state that the ocean will eventually return to normal.

Trivia

 * The acidic of the water on this planet mirrors that of water that Samus found in the Chozo Ruins. It was only by destroying Flaahgra, a Phazon-corrupted plant and the source of the toxicity that she was able to purify the spring.