In the last decade, the U.S. Navy has had ample examples of how vulnerable American ships are to modern diesel-electric subs. In that time, French Daphn class, German Type 209, and Swedish Collins class (used by South Africa, Chile and Australia, respectively) have all gotten close enough to American carriers to hit them with simulated torpedo attacks. Russian and Chinese subs now have the technology to build subs just as quiet. During the 1982 Falklands war, a German built Type 209 Argentinean sub evaded a month of British efforts to find and sink it.
There are some 40 countries that have a combined force of over 300 diesel-electric subs. Most of these countries are either American allies, or are using older, noisier, type subs. But countries like North Korea, China and Iran are all benefiting from the Russian quieting breakthroughs of the last two decades. The U.S. navy got complacent about subs over the last half century. Russian subs were not only noisy, they were also manned by poorly trained crews. Russian nuclear subs were particularly noisy and easy to find. American ASW (Anti-Submarine Warfare) efforts were rarely challenged.
The U.S. Navy has not developed an effective ASW force since World War II. Lots of attempts since then, but nothing that really worked. This does not count the cooperative Russians, who built noisy boats that we could track. But all through that period we knew that the quieter diesel-electric boats of our allies were very difficult to track. Fortunately, the Cold War ended just as the Russians figured out how to make quiet subs. The Russians still have problems with poorly trained crews, but the Chinese are trying to solve that one. The Chinese sub force could thus be an ugly surprise in any future war. Quiet Chinese boats (like the new Russian Kilos they are receiving), and well trained crews, can easily be the end of American carrier aviation.
The most serious damage done to the U.S. Navy since World War II, occurred during the 1980s, when Soviet intelligence efforts developed two well placed agents in the U.S. Navy (John Walker and Jerry Whitworth). This spy operation revealed to the Russians how noisy (and easy to detect) their subs were, and what needed to be done to make them quieter. In two decades of effort, the Russians have made their subs so quiet that they can only be detected a few kilometers away by American sensors. In the 1980s, American subs could usually spot their Russian counterparts hundreds of kilometers away. Russia is now selling this quieting technology to China.