The rats, no, since they never specified where exactly they would be beyond 'in the vents'. I don't even have an educated guess for that.
As for what lies below, I have the manhole in the basement (and in the updated version the furnature piled on it; bookshelf, cabinet, 2 chairs), but we don't know what is down there besides a 'tunnel'. Maybe when they get around to answering the questions of what is down there I will add it.