Pic became too damn big. I'll fix it once I'm able.The left and right rectangles are two really big halls. There were signs that there were once walls there to separate sections of the halls but they're obviously gone now.
The topmost rectangle is where Neil found the furnace. Half of it still has a roof and covers the furnace. The other half is... gone. Or just sitting as rubble.
The small rectangles over the lines indicate doors
The darker lines are the stairs that lead to the 2nd floor. It was probably wise not to step on them before repairs are done.