The plastic "belly pan" or undertrack beneath your engine isn't just there for stylish good looks. It's also there to help air go where it's supposed to go. And over the years, they get pretty tired looking. There are some aftermarket options out there, but the factory one still does the best job of actually controlling airflow.
This is for all NA and NB models except the MSM. It can be made to fit the MSM with some modification.
Mazda part NA01-56-111F.