[Update]: Subnautica: Below Zero has now launched from Early Access, and without any sort of major technical issues!
Unknown Worlds announced back in August that they are working on a chilling standalone expansion - Subnautica: Below Zero. As you might expect from the name alone, the upcoming expansion will offer a brand new storyline set in the Subnautica universe, and will take place in the ice-bound region of planet 4546B after the events of the first game. Other than those little tidbits of information, however, we are almost entirely in the dark about Subnautica: Below Zero.
With that in mind, I am very glad to say that Unknown Worlds has now come out to announce that Subnautica: Below Zero will be heading to Early Access next week, on January 30th! The initial version is expected to be both unpolished and lacking in content, so if do decide to grab it as soon as it becomes available, make sure you're doing so with the intent of providing the developers with some much-needed feedback. Personally, I'd recommend waiting until the game is fully finished as that will give you the best possible experience, but I can definitely understand why some of you might be tempted to get in there as soon as possible.
Concept art showing what the surface will look like
I can't tell you much more about Subnautica: Below Zero as the developers are still being very secretive about it, but if you would like to follow their progress moving forward, you should go and check out the official Favro board. It's essentially a list of features and modules they're currently working on, so while it might contain some spoilers, it will give you a pretty decent idea of the shape Below Zero is currently in, as well as what the team is planning for the future.
Once more information becomes available, most likely on January 30th, I'll make sure to let you know as I'm quite curious myself.