How to make subnautica-like terrain

The title speaks for itself I would say, how can I make terrain that sort of resembles that of subnautica's? And if anybody knows, how did the developers of subnautica make that terrain?

 

There has been at least one GDC presentation (and I think many) about Subnautica. Definitely do some youtube searching.

 

Subnautica uses a voxel-based terrain system. That said if I were recreating the game I would create the terrain using traditional modeling tools and completely skip the voxel approach as I'm fairly certain you can't deform the terrain in the game world and voxels have higher overhead than a mesh.

https://unknownworlds.com/subnautica/terrain-data-format/

 

This might be a useful start.