In early 2015, Security Researchers from Ben Gurion University in Israel showed a theoretical attack on airgapped machines called BitWhisper.
The attack requires two machines to be physically infected first, and immediately adjacent to eachother within a server rack. The first machine is the air-gapped one, the second being a networked server immediately adjacent. The air-gapped server then very slowly (~8 bits per hour) passes information from the air-gapped server to the other infected server by increasing and decreasing the temperature of the server which is then interpreted by the networked machine. From there the data could be passed anywhere.
So far however, BitWhisper is only a theoretical attack.
Original BitWhisper whitepaper.