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These computers are connected through physical cables that exist primarily underground. To get internet from continent to continent, they also go under the ocean. The undersea cables are the veins that information flows through to circulate, similar to the veins of the human body distributing blood. Click here to see a visual map of these cables and who owns them.
Know all this? Check out this Stanford White Paper which goes more in depth.
internet + infrastructure; internet + architecture; internet + undersea cables