Why do basements stay damp even without leaks?

My basement doesn’t have visible leaks, yet it still feels damp most of the time. The air is humid, and there’s a slight musty smell. I’m trying to understand where the moisture is coming from. It could be condensation, but I’m not completely sure. I’ve been thinking about possible solutions but don’t know where to start. It’s frustrating when there’s no clear source of the problem.