Air gaps can also be bridged by using radio or sound waves. There could be a bunch on non-networked computers in a secure lab all talking to each other. This assumes trojaned hardware and/or operating system software in the systems that can send and receive data and commands.
Finally, technology such as Morse Code is still useful in these scenarios. Dits and dahs. Zeros and ones. That's all you need to be able to send and recv data.