Shortwave radio with big-ass yagis and dipoles, aimed at each other in point-to-point links. Very low data rates, carrying custom messaging protocols, but also measurably less latency on critical paths such as NY-Chicago or London-Tokyo.

There is a big crossover in tech with shortwave band military over the horizon communications systems, and also serious ham radio enthusiasts who use big yagis and antenna rotators to talk to each other from opposite sides of the planet.

Things like chained 6 and 11 GHz microwave between NY-Chicago, which was the "hot new thing" in HFT 7-8 years ago to achieve better than fiber latency on that route, seem quaint now.

Sorry, used wrong word. "Skywave" is a better term.