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): <a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10979452" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10979452</a>