Why is a magnetic compass not useful for navigating a submarine?

A magnetic compass is not useful for navigating a submarine primarily because submarines operate underwater, where the Earth's magnetic field can be distorted by the surrounding water and the metal structure of the vessel itself. Additionally, the compass relies on detecting magnetic north, which becomes unreliable due to the varying magnetic influences underwater. Instead, submarines typically use inertial navigation systems, GPS (when surfaced), and other advanced navigation technologies to determine their position and course accurately.