The internet’s largest and most important — and yet unsung — champions are the privately-owned submarine cables that orbit the Earth. Terra firma links between cities and cables that run alongside roads and into houses and officers are certainly impressive — and without them we wouldn’t have an internet! — but sinking a cable into the Pacific, Atlantic, Indian, and even Arctic Oceans requires a billion-dollar logistical feat that requires months or even years to enact.

Across these cables, which span distances of up to 13,00o km (8,000 miles) and have total lengths over 21,000 km (13,000 miles), terabits of information squirt from one side of the planet to another. To get from London to Tokyo, your packets can traverse Europe, the Mediterranean, the Red Sea, the Arabian Sea, the Indian Ocean, and finally the South China Sea — or they can hop across the Atlantic, the entirety of continental North America, and then long haul over the Pacific. For a complete (and interactive!) view of the world’s 188 planned and active submarine cables, check out Greg’s Cable Map or TeleGeography.

Submarine cable cross-section (from Wikipedia)These cables are just three inches thick, carry just a few optic fibers, and have total capacities of between 40Gbps and 10Tbps, and latencies that are close to the speed of light and just a few milliseconds in duration. These cables shouldn’t run out of capacity any time soon, too: we’re already at the stage where we can send 40Gbps over a single fiber, and graphene optical switches should expand the total capacity of submarine cables (and the terminating routers) into the petabit- and exabit-per-second range. In the image to the right, #1 is polyethylene, #2 is mylar tape, #3 is stranded steel wires, #4 is an aluminium waterproofing layer, #5 is polycarbonate, #6 is a copper or aluminium tube, #7 is petroleum jelly, and #8 is the optical fiber itself.

As far as laying a submarine cable, specialized cable-laying ships must be used — and again, when a cable is broken (usually by a trawler, but sometimes a whale!), another special ship must be used. This generally means that laying a cable is logistically challenging and very expensive — and when a cable breaks, it sometimes isn’t possible to fix it immediately if a cable-fixing ship isn’t nearby (and for this reason, most submarine cables use a ring topology in case one stretch is broken).

It’s also amazing to consider that the first trans-Atlantic cables were laid in the 1860s, and trans-Pacific cables followed in the early 1900s. These cables were incredibly low-bandwidth — repeaters didn’t exist yet, so the only way of getting a signal across the pond was by upping the voltage and creating a very noisy link — but by the early 1900s, the British Empire had already connected up most of the continents (see below). It’s also worth noting that the only continent that isn’t wired into the internet is Antarctica; the temperature of the Antarctic Ocean is too low, and the movement of the ice shelf by up to 10 meters per year is tricky to overcome.

Eastern Telegraph Co. (British Empire) cable network in 1901

Finally, we would be remiss to ignore geostationary, orbital communications satellites. While satellite data links can be in the gigabit range, the high latency of bouncing a signal through a point that is rather far away (35,000 km, 22,000 miles) makes them unsuitable for many consumer internet services. This same latency will pose some big problems when we start colonizing other planets and need to create an interplanetary (or intergalactic) internet, or galnet for short.

Read more about optical submarine cables on Wikipedia