Cracked and settled floor in lower lever bi-lever home.

[Iaroslav Demydovych]

Fellow BP members I'm looking at a really good deal to flip it just has one issue I'm not sure what to do with. Floor in lower level is settling or settled about 2.5 inches, it even feels hollow under. Its cracked and sagged in different directions. Outside brick and blocks look perfect, no cracks no sagging or bending in metal siding. What do you do in situations like this? Why would it feel hollow underneath? 

This one wall has brick inside behind the fire place, you can see the gap is about 2.5 inches.