Seamonsters

The third Degasi base sits in the Deep Grand Reef, roughly 500m down - follow the story waypoints (or the giant cove skylights south-west of the Grand Reef) and mind the resident Crabsquids on the approach. As with the other Degasi bases, the unlock comes from the PDAs: enter and hoover up the logs inside.

Whilst you're down there, pick up a Cuddlefish egg from the base floor for "Man's Best Friend" - saves a second trip to an unfriendly neighbourhood.