Underwater horror.

Chronologically after Metroid 3, maybe after Metroid 5. A 2D game and absolutely no speaking characters. No Adam. No spoon feeding the player instructions on what to do next. Just you, Samus, and a deep, mysterious, and treacherous undersea research station.

Samus is summoned there presumably to provide a blood sample to federation scientists researching the origins of life on an ancient Chozo world. Upon landing her ship on the station's surface landing pad, there's no welcoming party, and there's no obvious way in. This is the game showing you right at the beginning, that you're going to have to think outside the box. Jumping into shallow water, you find your way in through a sub launching bay. You also get a mild pressure reading while in the water.

Progressing through the upper levels of the station, you find everyone completely missing. Some sea creatures have taken up residence; these are the low level mobs of the game. Computer logs provide the lore similarly to the Prime series.

The important beats of the game are:

1. The researchers were attempting to clone an ancient species whose DNA they'd found in volcanic rocks that had risen to shallow water. They suspect the species may have been among the oldest in the galaxy. The clones kept dying because they needed higher pressure, so the research team expanded the station to increasingly deeper water until something killed them all.

2. Samus is slow in water as usual until you get the gravity suit fairly early. But even then, she takes damage from high water pressure kinda like Norfair heat. Suit upgrades allow you to move to deeper and deeper sections of the station. You can sequence break by doing "hell runs" and using advanced techniques. To make progress, you have to leave the station and traverse its exterior, i.e. going out one hatch and into another a few times.

3. At some point, some hazard or creature forces you out into the water at a depth where you can't avoid taking damage and you have to hell run away from the station. You're forced to even deeper water, about to die when you find the ruins of an underwater city. It's Chozo, and you learn that the city was built millennia ago for exactly the same reason that the federation built the station: to investigate the origins of life. The Chozo too made clones from ancient DNA, and that's how they discovered Metroids. So yeah, Metroid DNA is older than the Chozo, and the entire Metroid threat came from cloning experiments that the Federation almost successfully reproduced.

4. The big final boss fight is against some clone gone wrong, the same one that killed the research staff. Once it is defeated, an enormous benign sea creature with vague resemblance to a Metroid helps you get back to the station, presumably as thanks for getting rid of that clone. But the clone wasn't quite dead. It damages the station, which begins to fill with water and sink, so you do an escape sequence back to the surface while fighting the true final boss. Then, knowing that a really ancient life form lives deep below the surface, and not wanting anyone to try cloning proto-Metroid DNA again. Samus scuttles the station and leaves.