Or, if that is too complex, there could just be a bunch of breadboarded ICs capable of networking. There actually are real-world examples of such machines exposed via telnet, e.g. http://www.homebrewcpu.com
And all the performance beasts, while surely indispensable, could just enjoy their air-gapped solitude. Are there any massively parallel supercomputing tasks whose results couldn't be summarized and reduced to mere text, which is not too hard to move over the airgap manually?
Video decoding? Games? You know, the major use cases for custom highly patented hardware design?