AIP: How does it work?

I have a brief explainer at the National Interest about AIP (air independent propulsion) submarines…

Over the past decade, air-independent propulsion (AIP) for submarines has spread rapidly around the world. The technology, which allows conventionally powered submarines to operate without access to outside air, has the potential the shift the balance away from the big nuclear attack submarines (SSNs) that have dominated undersea warfare since the 1950s, and back towards small conventional boats. In global terms, this might again make submarines the great strategic equalizer; small, cheap weapons that can destroy the expensive warships of the world’s most powerful navies. Does this mean that the United States should invest in these kinds of boats? Probably not.