In the end, the internet is made up, in large part, of actual wires. There are ‘nodes’, like where the Trans Atlantic cables come ashore, that are huge vulnerabilities.
Has anyone ever shut down the internet of their opponent by physically cutting the wires?
If you can read this, then not in the last half hour, they haven’t. 
Actually, this is a non-sequitur.
The Internet is, by deliberate design, a widely distributed network with lots of inter-connectivity. If any specific node is attacked, it might knock some part of the net disconnected from the rest, but there is supposed to be enough redundant inter-connectivity that messages can be re-routed around the broken point.
The early Internet, originally called Arpanet, was a project of the Defense Advanced Research Projects Agency, going back all the way to the 1960’s. The whole idea of the design, right from the start, was to design a wide-scale network that could withstand any outages (including enemy attacks) on any particular points in the network. There are intentionally no insurmountable bottle-necks.
It looks like Mubarak took out the Internet in Egypt, just to annoy his people:
No physical damage, but it was an active attack to deny internet service to a nation.
There’s also this event, which may or may not have been an active, physical attack on the internet:
Mother nature has done this many times, that nasty old hag.
As mentioned up-thread, redundancy is one of the primary purposes that the 'nets are designed as they are. Most of the “dirty” internet uses OSPF (Open Shortest Path First) routing, which means if there’s high latency on the ideal path the core routers will find a way around it. This can (and does) increase latency, but isn’t the end of the world.
To use, say, North America as an example, if you cut the trans-Pacific cables, we could re-route data through South America or directly to Europe, across Asia, and you’d still be able to hit Japan.
But your pr0n would be frustratingly slow.
I would strongly recommend Dark Territory, by Fred Kaplan. It’s an excellent history of cyber warfare, cyber security, cyber everything, and delves into exactly your question. As you might expect, when the FBI, CIA, NSA and Department of Defense (to say nothing of all sorts of other alphabet programs) are involved, the answer is not a simple yes or no. Also check out the YouTube video of the Aurora Generator Test, as an example of how computer specialists illustrated just how easy it is to crash the electric grid. A very interesting and informative book, but it may keep you up at night. Sweet dreams. xo,
C.
I recall some new item about some people being arrested off Alexandria apparently attempting to cut an undersea cable a few years ago.
Also, when I flew into New York about 2 weeks after 9/11, none of the ATMs at JFK were working. Apparently a lot of the communications network went through relays in the WTC.