"For many decades, U.S. officials have been keenly aware that, because of the consulate’s proximity to Silicon Valley, educational institutions such as Stanford and Berkeley, and the large number of nearby defense contractors and researchers — including two Energy Department-affiliated nuclear weapons laboratories — Russia has used San Francisco as a focal point for espionage activity.

[..] One former senior counterintelligence executive, for example, recalled the “disproportionate number” of science- and technology-focused Russian intelligence officers based in San Francisco, some of whom were experts in encryption and were tasked with identifying new developments in such technologies in Silicon Valley.

[..] Over time, multiple former intelligence officials told me, the FBI concluded that Russia was engaged in a massive, long-running, and continuous data-collection operation: a mission to comprehensively locate all of America’s underground communications nodes, and to map out and catalogue the points in the fiber-optic network where data were being transferred.

[..] Russian operatives appeared to be actively attempting to penetrate communications infrastructure — especially where undersea cables came ashore on both the Atlantic and Pacific coasts. They were “pretty sure” said a former intelligence official that, on at least one occasion on land, a Russian operative successfully broke into a data closet (a telecommunications and hardware storage center) as part of an attempt to penetrate one of these systems.

But what was “really unnerving,” said the former senior counterintelligence executive, was the Russians’ focus on communication nodes near military bases. According to multiple sources, U.S. officials eventually concluded that Moscow’s ultimate goal was to have the capacity to sever communications, paralyzing the U.S. military’s command and control systems, in case of a confrontation between the two powers. “If they can shut down our grid, and we go blind,” noted a former intelligence official, “they are closer to leveling the playing field,” because the United States is widely considered to possess superior command and control capabilities.

[..] Suspected Russian operatives were appearing in places that had recently been, or were later, part of Russian flyovers. [..] If they appeared afterward, U.S. officials believed that the Russians had identified a potential object of interest (such as a fiber-optic node) and wanted in-person confirmation on what previously been identified during a flyover. There is simply “no substitute for someone literally going to locations and recording GPS coordinates,” said the former senior counterintelligence executive. “From 30,000 feet, you’re not necessarily going to have accuracy if you’re pinpointing a portal.”

[..] Russia has aggressively exploited its diplomatic presence in San Francisco for decades, and the United States has historically responded in kind. In 1983, for instance, the State Department issued new guidelines forbidding Soviet diplomats and journalists from visiting Silicon Valley."

https://foreignpolicy.com/2017/12/14/the-secret-history-of-the-russian-consulate-in-san-francisco-putin-trump-spies-moscow/