What is it that causes the "musty smell" in a basement?
Dampness and humidity generally.
Dirt and dust and moisture.
The rotting corpses buried under the floor. Mold, from to much moisture. Get a dehumidifier, check to see your basement is sealed well (even the walls) & if you have a sump pump make sure that it is operating correctly
Fecal matter of bacteria feeding on mold. It is natures way of protecting us from dangerous compounds which can form from rotting substances.
Moisture. If you can't afford a dehumidifier, get the product called Damp Rid. You can find it at Home Depot.