These subs are large, metallic, full of electric equipment, and heavy. They have magnetic and gravitational effects on their surroundings. They also have elec

These subs are large, metallic, full of electric equipment, and heavy. They have magnetic and gravitational effects on their surroundings. They also have electromagnets to help hide their magnetic signatures. A network of very sensitive satelites with very precise clocks could be used to detect gravitational anomalies -- maybe, I'm not sure what kind of precision would be needed, or if that is achievable.