Calling the results of the earthquake and resulting tsunami that happened in Japan a catastrophe would be to understate the seriousness of the event. It was Japan's worst earthquake ever. It was one of the worst in recorded world history.
A vast portion of the northeastern part of Japan's main island was virtually swept clean by the water, leaving little but rubble, death and thousands of terrified people in its wake. But if there can be a bright spot in this unimaginable tragedy, it is that Japan's part of the Internet has remained intact.
Despite the loss of nearly half of its undersea cables, and despite the lack of power in the regions most seriously affected, the Internet outages were brief when they happened at all. Most of Japan never had an Internet outage at all, although there were a couple of hours when access was slower than it might have been otherwise. So the question is, with all of this damage, how can it be that the Internet was hardly affected?
The answer, of course, lies in two places, the first being the U.S. Department of Defense. I suspect that few people now frequenting the Internet realize that this vast network of networks started life as a DARPA project. The goal of the project was to design a network and a networking protocol that could survive a nuclear attack on the United States. This project, which came about when the Cold War with the old Soviet Union was cold in name only, was intended to give DoD facilities a data network that would be hard to put out of service.
So the solution was a protocol called TCP/IP, which divides information into small discrete packets, and finds a route for the destination of each packet according to what's available at the time. The final design was so successful that it proved impossible to keep from general use. Eventually the DoD gave up running it and turned its administration over to commercial interests. But the DoD still uses the Internet, and it still demands that the current network keep its survivability intact.
As a result, the Internet does what it has always done best, which is to find a way around damage and deliver the data packets it's supposed to deliver. But in the case of Japan, there's more to this story. After all, if a disaster takes out enough cables and enough routers, there might not be a path available.
The Japanese government and its Internet service providers, however, developed what they called a "dense mesh" network. This means there are many possible routes between places on the network and the extra network connections make it relatively easy for routers to find a pathway.