That “musty smell” that many people think of as being indicative of a regularly functioning crawl space is actually not an intended crawl space feature. The musty crawl space smell actually occurs because of mold growth, which feeds off moisture to grow.
This is exactly why a “musty smell” should be a sign that you need to fix something. If your crawl space smells musty and stale, there’s something wrong. You can fix it and avoid potentially permanent damage from mold and moisture, but you have to take the first step. Don’t wait to check out your crawl space; there’s a decent chance that there’s something happening you can fix.