B777 Undifferentiated or Other Model
Air Traffic Control Fully Certified
Communication Breakdown; Time Pressure
Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Executed Go Around / Missed Approach; Flight Crew Returned To Clearance
Aircraft
I just plugged into position. The Controller I was relieving told me to watch out for Aircraft X because he seemed kind of off. Aircraft X was cleared for the ILS to Runway XXL. After working the other airplanes and settling into the position; I noticed the Low Altitude alert go off on Aircraft X. I told the pilot Low Altitude and the altimeter. He continued a steady descent through 1;300 on an 11 mile final. I kept telling the pilot he was too low and to climb. There was a language barrier; but he was responding 'Roger' to me.He kept descending all the way down to either 600 or 500 feet on an 8 NM final which even though we were east traffic over Ocean is extremely low for a B777 to be at on an 8 NM final. After the Tower kept getting in my ear while I was trying to repair and control a dire situation; the plane didn't start to climb back up until they were at the final approach fix. In that state; they were unstable again; so I sent the plane around. I don't know what happened with the pilot and their equipment but I believe that since they were over the water that something must have gone wrong; whether it be perception or equipment malfunction. They seemed like nothing was wrong and were unaware that they were too low. Our operation is usually west traffic so I don't know if these pilots were just caught off guard with our un-ordinary operation. I believe this is just pilot error and mechanical malfunction. I don't know how this could be fixed.
TRACON Controller reported an aircraft descended to an unsafe altitude while not responding to ATC.