Developer Unknown Worlds has revealed first details of Below Zero, its long-teased arctic expansion for superb underwater survival adventure Subnautica.

Below Zero is described as a "new chapter" in the Subnautica universe, and unfolds across a previously unseen ice-bound region of planet 4546B - the ocean world that players explored during the base game.

Unknown Worlds says that it's still developing the concept of Below Zero, but that the expansion will retain the core mechanics of the original Subnautica, including base building and open-ended exploration. These will be joined by new elements, with the developer currently considering the likes of thermal management, and more voiced dialogue for the central story.

Concept art on the announcement page also teases arctic adventures both below and above the water's surface, and there's a glimpse at a hovercraft-style vehicle careening across the ice.

Notably, Unknown Worlds refers to Below Zero as a standalone expansion, and says that it will be launching an "unfinished, early version" into early access "in the coming months". The developer will then begin the process of releasing consistent content updates, "carefully crafting the game based on your feedback - Just like the original Subnautica".

That's all the formal information that Unknown Worlds has shared so far, but it does note that a few additional clues (and spoilers!) can be found by perusing the game's development log on Trello. There's currently no word on an anticipated final release date for Below Zero or the platforms it will launch on, but it's almost certain that it'll follow a similar path to its predecessor, arriving on PC, PlayStation 4, and Xbox One.