To avoid a repeat crash like a Boeing 737max8, submarines can be equipped with sensors acting at long distances like a lobster's antenna or catfish's whiskers, snake's tongue or slime mold's feelers to map out immediate surrounds and continuously refresh the map.

> Did they navigate into one of the Chinese man-made islands?

This was my pet theory as well. Man-made sea mounts would be inherently uncharted and would be a great defense against adversary submarines.

It could be Chinese stealth underwater drone.