Bolted shut from the outside in case anyone had any ideas about escaping
The support ship lost contact with the submersible at 1 hour 45 minutes from launch, short of the 2 hour 15-20 minutes time to reach the wreck. If it was not safe below 3000 meters, was not upgraded to 4000 meters but made subsequent trips to that depth of the Titanic, they were just lucky. Their luck may well have run out this time.