Related, interesting HN discussion a month ago:<p><a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10810390" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10810390</a><p>Submarines can alter their magnetic fields, but gravitational fields are a good bit more tricky, and can apparently also be exploited for detection.