Fix a smelly basement?

I just moved into my grandparents basement and it has this unpleasant smell to it. I guess you can call it musty mixed with old people but I've sprayed Oust a bunch of times and I have Febreeze air fresheners but they just seem to cover or blend with the smell rather than fix it. I know dampness can be an issue but they've had a dehumidifier running down there for a long time now and the smell is still there. Does anyone know what I can do to fix this issue? I don't want all of my things to smell like that.