CRABS. Centurion Robotics Autonomous Benthic Scouts.
The CRABS originally designed to sit on the ocean floor and monitor enemy fleets during the Last War. Repurposed as autonomous research machines. They discovered the Dark Reef.
Dark Reef Research Outpost 4. Entire staff is missing, except Ted, who was out cleaning the air scrubber.
First ruins discovered inside the Dark Reef. They're not human in origin, but they contain human-like structures like doors and windows. They also appear to be land structures that were suddenly submerged.
Ocean floor mining facility "Novskorad", 4km from the Dark Reef, was completely abandoned. No trace of the 35 human workers.