Oxygen

Diving is the most important thing you do in Subnautica. Oxygen (O2) is needed to breathe underwater, and the player only has a very limited reserve. The Oxygen automatically replenishes whenever the player comes back to the surface, enters a submarine or a seabase. Oxygen can also be replenished by swimming through bubbles produced by Purple Brain Corals.

The HUD indicator for the player's Oxygen supply is on the left side of the screen. When the Oxygen reserves are empty, the screen slowly fades to black and after about 6 seconds, the player dies and respawns.

In all non-Hardcore game modes, there are alerts for when the player has 30 seconds and 10 seconds of Oxygen remaining. However, the 30 seconds alert doesn't play until the player has acquired tanks to boost their Oxygen capacity beyond the default values.

The deeper the player is, the more Oxygen is used up, meaning that the reserves of Oxygen deplete faster. This can be counteracted by equipping a Rebreather.