The coastal ocean between southern Japan and Taiwan (like the waters between Taiwan and China's mainland) are shallow, by naval standards. More importantly, this shallowness affects sonar detection efforts due to the lack of "thermocline layers" of differing temperature currents, which are present in deeper, colder waters. Only active audible "pinging" sonar is reliable in such waters, but like tracer bullets fired at night, point directly to where the fleet is and can be used as a homing beacon for torpedos,...so active pinging is usually a last resort method these days.

The part of a nuclear sub which makes noise and can be tracked (if your equipment is sensative enough) is the coolant pump. Diesel/electric subs are like a Honda Prius - they make noise when they run their diesels (which is only on the surface), but are ultra-stelthy quiet when running the electric motors. Quieter than even nuclear subs. If you never leave the coastal waters of your home country (where it is shallow and hard to track you), and you never range far enough to require surfacing to run your diesels to charge your batteries,...diesel/electrics are ideal, and one sixth the cost!

The U.S. Navy knows all this of course,...it's why they truly fear ever having to back Taiwan against China;...because they would have to enter the Taiwan straights, and likely lose many ships.

The new generation of "active" sonar (instead of pinging), which has been the reason for most whale and dolphin beachings around the globe for the last decade, was specifically designed to better help the U.S. Navy secure its "battle-space" in shallow seas. I am certain that this exercize was (in part) meant to test that.

Thus, this incident is a larger failure than most realize, and calling it "as disturbing as Sputnik" is not an exageration.