Subnautica: Below Zero is an upcoming game that acts as an expansion to the story of Subnautica. Below Zero will be purchased separately, and as such will not require the original game to play. The arctic will be the setting of the story, which will feature a different protagonist rather than Ryley Robinson (not an Aurora survivor), and takes place in a different map in a time after Ryley Robinson has escaped Planet 4546B. The website can be visited here, and the Favro.
Subnautica: Below Zero Wiki
A separate wikia exists for Subnautica: Below Zero. A link can be found below.