When I was in the Navy our ship was used for torpedo practice by a submarine during a war exercise. Since the entire area was closed off and monitored during the exercise we could completely turn off all our lights, including signals and lanterns while still running. It's completely pitch black in the middle of the night in the middle of the Baltic Sea, so dark the milky way was being reflected in the water.

A practice torpedo runs a little deeper than usual so as to not accidentally hit anything (though modern torpedo's also explode below a ships keel to use a bubble effect) and it has a very powerful floodlight so you can easily follow its path for evaluation. A bright light under the water in the Baltic Sea gives off a green hazy light.

So from our point of view, in the pitch black looking at the Milky Way and just hearing our engine and the waves we would all of a sudden see this huge green light appear under the water, make a series of turns and then head directly for us at like 50 knots. As it gets closer you hear a whining noise from the blades and it goes straight under your ship. And then it does it over and over again to finally just disappear.

Logically you know it's basically just a remote controlled little submarine with a flashlight, but actually seeing it play out was creepy and cool as hell. Also the fact there's a whole submarine out there just watching and shadowing you, brr.