I wonder what exactly "isn't connected to the internet" means. Every time we build a military installation, we lay a dedicated cable from its nearest neighbor? That can't be right.

Maybe it's mostly using satellite connections?

Seems like there'd be many times (particularly in the US) where tunneling an encrypted connection over a civilian ISP would be worlds better than the next best option. Maybe not even at the IP level, but a dedicated wavelength on some existing AT&T fibre, for example.

Minor correction: it technically is connected to NIPR ("the internet") via cross-domain guards. Most information would go one-way, low to high, but very specific applications do warrant information going from Secret to unclassified in a very controlled manner, especially when it is time sensitive.

It is actually an incredibly interesting topic, but it is hard to get much information on the specifics online (I assume many details are classified or at least SBU).