The Alien Thermal Plant in Subnautica is inside the Lava Castle in the Inactive Lava Zone. You usually reach it by descending through the Lost River into the ILZ, then navigating to the large volcanic mountain in the center of the zone.

Fast route

  1. Go down to the Lost River.
  2. Continue into the Inactive Lava Zone.
  3. Look for the big central volcanic structure called the Lava Castle.
  4. Find one of the higher cave entrances on the castle’s sides, marked by alien architecture.
  1. Bring Purple Tablets to open the locked doors inside; you need two to fully explore it.

What to expect

  • The Thermal Plant is the smallest main alien base in the game.
  • It’s guarded by dangerous fauna in the area, so a strong vehicle and good depth upgrades help.
  • Behind one locked door is the Blue Tablet , which is needed later for the Primary Containment Facility.

Practical tip

If you’re having trouble spotting the Lava Castle, search the middle of the ILZ for a tall, smoky volcanic mountain rather than a flat facility; that landmark is the key clue players use to find the entrance.

TL;DR: head to the Inactive Lava Zone , find the Lava Castle , and use Purple Tablets to enter the Alien Thermal Plant.