The Stuxnet virus was designed to get inside the Iranian nuclear facility at Natanz. The idea is to isolate a PC from the big bad Internet and any other communications networks. Air gaps have been around for many years, and have found their way into military and intensely secure installations. But they aren’t foolproof, and even the best planned air gaps can have their weaknesses. An all-in-one printer can be used to flash lights from their scanning bed if the cover is left open.”]
Source: https://www.csoonline.com/article/2858751/how-to-bridge-and-secure-air-gap-networks.html