Air gap
A system with no physical connection to other networks. The strongest isolation available — and considerably less absolute than it sounds.
It's used where consequences are unacceptable: military networks, control systems in power plants and industry, certificate authorities' root keys, offline backups and cold crypto wallets. Stuxnet demonstrated the limitation — the worm reached Iranian enrichment facilities via USB sticks, because data has to get in and out somehow anyway. That's the recurring weakness: the human carrying the file across. Research has additionally demonstrated exfiltration over acoustic signals from fans, electromagnetic emissions from cables, blinking LEDs and even heat exchange between adjacent machines — slow, but enough for keys. The real drawback is operational cost: updates and maintenance become manual labour.