You control the Earth but who controls the skies?

Earth has seen the collapse of the machine-controlled society and the return of humans to the surface of the planet. Explorers have uncovered a previously unknown mega-city that is well protected from scouting incursions. All attempts at penetrating the border have been met with destruction as the line literally goes silent. Once again the call goes out to the Ravens, mercenaries hired by the government to break through the silent line and discover the truth behind the enigma.