It's remarkable that just one single cable access (NIGELLA in Cornwall) provides almost one tenth of everything NSA collects from internet cables. This also means that besides a large number of small cables accesses, NSA can only have access to just a few more cables with a similar high capacity as FA1 and FEA.
“Spycraft” relates in fascinating detail Operation CKTAW, one of the more elaborate technology feats of the entire Cold War. Radio technicians in CIA’s Moscow station became curious about microwave transmissions audible during heavy rains. They proved to connect a nuclear research lab in Troitsk, a closed city outside of Moscow, and the Ministry of Defense. CIA had just commenced monitoring when the Soviets discovered the technical glitch and shut down the transmissions.
Soon thereafter, a KH-11 reconnaissance satellite revealed that the Soviet military was laying communications in a trench between Moscow and Troitsk, along a major thoroughfare. Ground examination showed a number of manholes along the route. Agents made several sneak-in visits to the manhole vaults and photographed the interior. A mock-up was built at “The Farm,” the CIA facility near Willliamsburg, and technicians from the National Security Agency taught officers how they could tap the Soviet cables.
The officer chosen for the mission was “Ken Seacrest,” as he is called in the book. (I inadvertently discovered the fellow’s real name while researching a book; the suggestion was made, politely but firmly, that I purge it from my memory.) “Ken” was assigned to the Moscow embassy as an active family man - pretty wife, two kids - who was enthused about Russian culture and the outdoors, and prone to frequent outings. KGB watched him briefly, decided he was harmless, and maintained only “light surveillance.”
So, one weekend Ken and family established a picnic beachhead in a park. He slipped into the woods, donned Soviet worker clothes and made his way, on foot and by bus, to the manhole. He climbed inside and stood thigh deep in icy water for two hours while he applied taps to the cables. He then rejoined the “picnic.”
The next several years, the Agency reaped a bountiful harvest on Soviet nuclear programs. Then CKTAW was betrayed by Edward Lee Howard, who learned of it during CIA training before defecting to the Soviets. A KGB counterintelligence office wrote later that the CIA’s power source on the taps lasted four to six months and could be monitored from three kilometers.
(Washington Times review of this book
I'm not sure what's surprising at this point. After all, all the domestic spying of the last NSA scandal happened in one room of one building. At some point, a flowchart develops:
1) Is there a massive communications network?
2) Does that communications network have chokepoints?
At that point the question isn't "is someone listening" it's "how many different agencies are listening."