Subnautica Walkthrough Pt 20: Kyanite Run - How to Gather Kyanite and Finish Upgrading Your Vehicles 

We'll drop down into the Inactive Lava Zone, mine some kyanite deposits with the Prawn, then return to the Tree Cove to finish crafting thermal reactor and final-tier depth modules.
Bonus: Demo of how to start an engine fire and use the Cyclops' fire suppression system to put it out.