Mold in sub-flooring can be difficult to be aware of
I think the before photo here can say 90% of what we are all thinking..YUK! This situation happens more often than you think. Water somehow managed to get below the surface and never dried out, so moisture and dark do work to bring about mold.
In this circumstance there were some cracked tiles in the kitchen and water would seep through from any activity, ironically even mopping the floor to clean it.
The after picture shows you that SERVPRO can get rid of the mold and make it look "Like it never even happened." Words we live and work by!