Surely you are thinking of Sonar?

No, this came up on Hacker News recently: https://news.ycombinator.com/item?id=10810390

Gravity for navigation, perhaps ... but I believe they use magnetic fields, not gravitational fields, to detect other subs (and also some rather sophisticated real-time degaussing to prevent said detection): https://news.ycombinator.com/item?id=10979452