Gradiometer as here: https://www.sciencedirect.com/topics/earth-and-planetary-sciences/gravity-gradiometers https://www.sciencedirect.com/topics/earth-and-plane

Gradiometer as here: https://www.sciencedirect.com/topics/earth-and-planetary-sciences/gravity-gradiometers https://www.sciencedirect.com/topics/earth-and-planetary-sci... We'd have difficulty in fitting LIGO in a sub methinks and seamounts are static and thus no G waves. You're right though, I should have been more specific, perhaps a more understandable description would be a mass detector or sensor. I reckon the reason we don't hear much about this subject and the reason it's likely classified is that's its primary purpose would be the detection of other subs, detecting seamounts a secondary benefit.