[XorNot]

With sonar. The submarine doesn't have to locate itself, it just needs to be active pinging and it can be triangulated.

Radio doesn't work underwater, but sound actually travels incredibly far (hence why military subs go all out with noise reductions).

Edit: I mean the real question is why the hell does this thing not have some type of system like this already? You know the upfront expected dive time, so set a couple of noisemakers on the exterior hull to go off after you exceed it.

[whythre]

I am not a marine noise expert, but it’s my understanding that noise transmits horizontally underwater way better than vertically. Since this sub is going really deep, getting the pings to go ‘up’ to a meaningful degree is going to be difficult.

[atoav]

This is correct, the different temperatured layers of water typically develope quite well defined borders, these border regions act like any border between mediums with differing densities: they refract and reflect waves passing through them.

[XorNot]

Right: but even in then, you'd still be able to drop a microphone down through the layers and rapidly survey the whole thing at once. Much easier if the target is making a racket.

[atoav]

Good point, that could be used to the searchers advantage.