This guide will let you know how to get to the mountains in Subnautica.
Starting from the coral tube sticking up out of the water, you have to head about 2 bumps to the left of the northeast, and you will reach the mountain biome.
It is recommended to have a sea moth with an Mk1-Mk2 depth module, perimeter defense, sonar, high capacity 02 Tak, a sealide, a knife, and a Beacon to mark the mountains to make coming back to it later easier.
The main threat you will encounter in the mountains is the reaper leviathan; you will need to avoid them as best as possible and use perimeter defense against them if they get close to you. The other threat is warper which can warp you out of your vehicles unless you’re in a cyclops, so if it does, you can either deal enough damage to it until it warps away or use perimeter defense against it.
The coordinates for this location are 1090, -265, 1215.
When you start to see thermal vents that look like this, it’s a good indicator that you are getting close to the mountains.