Fourteen Thousand Leagues Under the Sea

The Primary Containment Facility is the bottom of the world: the Lava Lakes, roughly 1,400m down, below even the Inactive Lava Zone. A max-depth Prawn (or very carefully driven Cyclops) gets you there; the facility's entrance wants a Blue Tablet - collected at the Thermal Plant (Thermal Activity) or crafted from its ion tech. The achievement pops inside, and what's waiting in the aquarium is the game's great reveal - the road to Optimal Health starts here.