The latest idea in detecting submarines involves using powerful lasers mounted on satellites to penetrate to much greater depths than previously thought possible. As far as I can tell, the lasers don’t actually zap the submarines, they just tell you where they are. This could still be a big deal because undetectable submarines with nuclear weapons are the ultimate deterrent. A first strike on an opponent’s land- and air-based nuclear forces can never succeed in wiping out the ability to retaliate, as long as the subs are out there. There is even a solid argument that a few submarine-based weapons are all the nuclear weapons a country really needs strategically. So if someone gets ahead in the sub-detection game, a first strike could become more thinkable.