Charts which get updated periodically mapping the sea floor. Ship's fathometer if the Navigator has a reason to believe we may come close to one. The majority of areas subs operate in are pretty deep. Far deeper than the sub can go. read more
Nuclear subs are already detectable at sea, and have been since they were invented, just like the submarines before them. It is a question of their stealthiness versus the sensitivity of the detection equipment. read more
The reason is because this method relies on knowledge of the density of the water column above the submarine, but this density is affected my many factors, and it is not the same at every point between the submarine and the surface. read more