WITHIN the last year there have been 16 so-called fiber cuts in the San Francisco Bay Area. According to the F.B.I., someone or some group has been going through manholes to sever fiber optic cables that supply telecommunications to large sections of the region, which is home to technology companies, academic institutions and Lawrence Livermore National Laboratory, overseer of the nation’s nuclear weapons.

Following each incident (usually occurring late at night and involving two or three separate fiber cuts) residents couldn’t make land or mobile calls, not even to 911, or send texts or emails. Hospital records in some instances were inaccessible. Credit cards and A.T.M.s didn’t work. And forget about Googling, watching Netflix or remotely turning on a coffee maker. (For security reasons, Lawrence Livermore declined to say how the cuts affected its operations.)

When we talk about the Internet, we talk about clouds and ether. But the Internet is not amorphous. You may access it wirelessly, but ultimately you’re relying on a bunch of physical cables that are vulnerable to attack. It’s something that’s been largely forgotten in the lather over cybersecurity. The threat is not only malicious code flowing through the pipes but also, and perhaps more critically, the pipes themselves.

Most worrisome are the throughways and junctures that handle enormous amounts of Internet traffic. It would be as if a major interstate highway or crucial interchange were closed and all the traffic was forced onto side streets. There would be gridlock, and some of those side streets might collapse under the weight. Data transfer would slow significantly or come to a halt, as has happened in Northern California.