Getting the Blue Key (3)

Start inside the submarine wreck. Swim to the bottom and press the buttons in the sequence X, O, X (from top to bottom) to open an emergency hatch.

Swim out of the submarine. Collect the Scuba Gear near the exit if needed.

Locate a large fracture in the wall. Grab the Pistol Ammo at the entrance and a Portable Medikit further inside, while avoiding Octabrains.

Find another fracture near the Octabrains. Destroy this wall to find Atomic Health.

Continue past more Octabrains and two boxes of Pipebombs.

SECRET 1: Upon emerging from the water, go to the land area with a babe and barrels. Use a Pipebomb on the barrels to open a passage to the outdoor area. The secret registers when you reach this land.

Emerge outdoors in front of a building. The building's entrance will open, revealing Troopers. Defeat them using the Chaingun Cannon.

Two Mounted Guns are on columns at the building's front. You can shoot the first from the Secret 1 passageway, but you must move in front of the building to target the second.

On a ledge opposite the building, three Troopers are present. Use an RPG blast to eliminate them.

Swim across the water to the entrance of the passageway leading to the ledge.

Passageway: Inside the passageway, pick up a Large Medikit. Climb the passage to obtain the RPG and the Blue Key.

While on the ledge, destroy Mounted Guns firing lasers from the left or right with RPG rounds.

Jump into the left hole where a Mounted Gun was located and open the back wall to find a small room with a message. On the opposite wall, another hidden door reveals a Holoduke (this does not count as a secret).

SECRET 2: Jump into the right hole where a Mounted Gun was located and open the back wall to find RPG rounds in the secret room.

SECRET 3: Go to the top of the submarine and step into the small burning indentation. This registers a secret. The spoils are located back inside the submarine: Body Armor, Atomic Health, and a Portable Medikit.