A few years ago a couple of big military submarines bumped into each. They were from different countries. Each received some damage, and nobody died, but neither was aware of what they had run into. An errant shipping container was the initial suspect. It wasn't until much later when they compared notes that they realized they had run into each other.
Submarines are like monsters sleeping in the deep. Unseen, hidden in the depths of the ocean, it's easy to forget they even exist. Should they awaken in bad mood they could wreak devastation that would make Godzilla look tame.
A fair portion of the world's economy is being siphoned off to support this insanity, but that's to be expected from a world filled with crazy people. While most people don't waste a minute of time thinking about them, there is a tight cadre of people who devote their lives to thinking about these machines, and a good deal of that thought revolves around detection and escaping detection. Detection is mostly done with sound waves, either with active SONAR (the ping you hear in the movies) or passive, which basically means just listening. Well, listening with a microphone, and then feeding the signal to some sophisticated DSP (Digital Signal Processing) systems to try and make sense of it.
Escaping detection basically means making the submarine quieter, which ranges from all kinds of sound deadening materials to sophisticated propeller designs. I remember there was some kind of brouhaha a few years ago over some fancy machine tools getting shipped to a communist country, tools that would enable them to carve super fancy, super quiet propellers for their submarines.
Some people are looking for ways to detect submarines from the air or even space. I found this paper from Australia that looks at several of the techniques being explored. Most of them seem to be right on the edge of being practical. They sound as esoteric as the scanner sweeps being done by Geordi or Will on Star Trek.