Fortune Magazine has an interesting article that details how the internet is cabled around the world – much of which travels underwater. The massive fiber-optic cabling forms the backbone of the internet.
From Fortune Magazine
If the Internet is a global phenomenon, it’s because there are fiber-optic cables underneath the ocean. Light goes in on one shore and comes out the other, making these tubes the fundamental conduit of information throughout the global village. To make the light travel enormous distances, thousands of volts of electricity are sent through the cable’s copper sleeve to power repeaters, each the size and roughly the shape of a 600-pound bluefin tuna. One rests on the ocean floor every 50 miles or so. Inside its pressurized case is a miniature racetrack of the element erbium, which, when energized, gooses the particles of light along like a waterwheel.