It could be caused by a lot of things like lightning, an impact, poor jack stand use... OR just the bottom paint
applied wrong, like over dirt or grease.
You haven't shown us a pic, or described the size. If it is only a few inches across, it could be nothing more than a paint blemish that can be sanded out and painted. OR it could require grinding down to good glass and a somewhat significant repair.
Is there internal damage at that location? If it was a bad impact or lightning, I would expect to see some issues internally. If the electronics
are okay, and the internal skin/structure looks fine, then the damage is probably not too bad... likely even 'cosmetic'. Though it needs to be fixed b/c something cosmetic can fester and become structural.