Diesel engines need air, submarines run underwater, so how do diesel submarines work?

If diesel engines need air for combustion, then how can submarines use them? Here's what you need to know about the clever engineering behind diesel subs.
The National Interest

Here’s How Anti-Submarine Planes Actually Work

Submarines are hard to find. They exploit ocean depth, acoustic complexity, and thermal layers to maintain low observability and high survivability. Diesel-electric submarines are especially quiet at ...