[defrost]

There are other ways, of course.

* air-gap malware can be designed to communicate secure information acoustically, at frequencies near or beyond the limit of human hearing.

* In 2014, researchers introduced ″AirHopper″, a bifurcated attack pattern showing the feasibility of data exfiltration from an isolated computer to a nearby mobile phone, using FM frequency signals.

* In 2015, "HELLONE", a covert signaling channel between air-gapped computers using thermal manipulations, was introduced. "BitWhisper" supports bidirectional communication and requires no additional dedicated peripheral hardware.

* Later in 2015, researchers introduced "GSMem", a method for exfiltrating data from air-gapped computers over cellular frequencies. The transmission - generated by a standard internal bus - renders the computer into a small cellular transmitter antenna.

https://en.wikipedia.org/wiki/Air-gap_malware

[h4ckerle]

Don't forget Stuxnet which crossed the airgap via infecting USB Devices.

https://en.wikipedia.org/wiki/Stuxnet

[defrost]

Not forgotten at all, deliberately unmentioned as I focused on crossing the air gap by means other than:

  The only way to move data to and from them is for someone to walk across the gap with physical media.

Stuxnet was walked across via physical media (USB drives).