Can someone explain the dark belly button thing?

I think a lot of us light skinned moms have experienced this. Your belly button turns dark during pregnancy, you assume it'll go away after birth, it doesn't, and one day you realize that you can scrub the darkness away!!!

What is going on with that? I've heard people say it's dead skin because your belly button stretches out (but then why is it darker than the rest of your skin?) I've heard people say it's amniotic fluid (how would that even be possible?) it's driving me crazy lol. Does anyone have a real answer?