Mapping the Internet: Undersea cables

FORTUNE — 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.

Once a cable reaches a coast, it enters what can literally be described as a lighthouse — a building known as a “landing station” that receives and transmits the flashes of light sent across the water (think Morse code, but in nanoseconds). The fiber-optic lines then connect to key hubs, known as “Internet exchange points,” which, for the most part, follow geography and population. The fastest-growing hubs are in the countries — former Soviet republics, mostly — where Internet penetration is not yet complete.