Find the Lost River

The Disease Research Facility is in the Lost River - the eerie green brine-river caverns beneath the map, entered most easily via the Blood Kelp Trench or Bulb Zone entrances. Follow the story waypoints through the northern arm past the giant skeleton and into the facility; walking in deep enough pops the achievement, and the story takes a turn.

Come prepared: a Cyclops or a well-modded Prawn/Seamoth (500m+), and don't taunt the River's ghostly residents. This is the doorway to the endgame chain - Thermal Activity and Fourteen Thousand Leagues Under the Sea lie beyond.