Science fiction author Neal Stephenson has compared earth to a computer motherboard, and it's not hard to see why that analogy makes sense; there are more than almost 300 separate cable systems, each stretching thousands of kilometres between continents.

"Submarine cables are absolutely critical to supporting the internet, carrying virtually all traffic back and forth across the world," says Kaveh Ranjbar, chief information officer at RIPE NCC, one of the world's five regional registries that help ensure the smooth running of the internet. "Their importance means that there are a lot of them, which in turn means the internet has redundancies in place in the event one section does get damaged."

Redundancy is why the internet has been so successful as a communications network despite natural disasters. An earthquake in Taiwan back in 2006 severed eight submarine cables in the area, catastrophically disrupting web access in Hong Kong. With half of its internet capacity wiped out, Hong Kong's banks were left floundering. With redundancy now in place, the same situation is unlikely to happen again.

The submarine cable network is delicate, it's easily damaged and it takes a long time to fix. The biggest problem is fishing trawlers; while deep-sea cables are mostly safe, in coastal areas cables are now being buried three metres under the sea floor to protect them from ships' anchors. That makes laying the cables - something that's done using specialised cable-laying ships - a far more laborious process, but it avoids the three weeks it usually takes to mend a break. However, there are now enough cables around the world that cities are rarely forced offline; if one cable is damaged, there are others ready to pick up the slack.

Submarine cables are not new. The first link, back in 1854, was a transatlantic telegraph cable laid between Canada and Ireland, which transformed international communications.

As well as reflecting its own history, the global map of submarine cables tells a fascinating story not only about the internet, but also about global development. It's like a road map of world trade; the big international shipping routes tend to mirror major cable routes, linking the US with Europe and Asia. Africa and South America are less well served.

Since they're hugely expensive to lay, cables have traditionally been placed between more developed countries, but new routes are constantly being added worldwide. One such project is Arctic Fibre, an ambitious new route that will pass through some of the planet's most inhospitable regions to connect Tokyo and London via the frozen Northwest Passage of northern Canada. "The new Arctic route will save time and money, but it's also important as it adds a completely new route, and this again enhances the redundancy of the overall system," says Ranjbar.

It will also bring high-speed internet to people in northern Canada and Alaska for the first time, a welcome by-product of linking two major global cities.

This is often the way with submarine cables, which are proposed as money-making ventures that link several major cities or nations. If they pass close to the shores of smaller towns or countries that can afford to build a spur to the main cable, great. Island communities often get completely bypassed, and have to depend on other ways of getting online.

"Telecoms firms aren't keen to connect them because a new cable could cost billions, whereas they'd only stand to gain a couple of thousand new customers," says Fisher, who adds that some of the world's poorest areas are also those most underserved by cable landings, most notably the east and west coasts of Africa.

Although cables to connect Africa are on the rise as its economy grows, there are still big gaps; if you don't have a coastline, accessing the huge capacity for fast internet offered by submarine cables can be difficult for developing countries.

Looking to plug the gap are satellites, which have a far smaller role in global communications that most people think. "A lot of people assume that, in this modern age, everything is handled by something far more elegant than huge lengths of cable spanning oceans, but in truth satellites only account for about 1 per cent of internet traffic," says Fisher. "Satellites are expensive to launch, they have relatively small traffic capacities, long round-trip times due to sheer distance to space and back, and they're subject to atmospheric conditions."

Satellites are also not quick enough to match the kind of web access cities such as Hong Kong are used to. "Satellites are an exciting prospect, but simply not cost-effective for developing nations at this time, and that's where the next billion internet users will come from," says Ranjbar.

The main reason is latency. There will always be about a half-second delay, which means video calling and movie streaming are unreliable.

Cables are by far the preferred way of expanding the internet, being both cost-effective and much higher capacity. "Satellites are great for areas where wired connections aren't an option, but they're slow and expensive, and are mostly used as a last resort," says Fisher. "There's plenty of room left on the ocean floors for more cables."

That few people know about the vast network of cables that stretch around the globe tells us much about how little connection we have with how technology works. "You turn on your phone or computer and it just magically works," says Fisher, and he's right - we all think about technology as magic. That's the selling point. "It's easy to assume that the whole chain that brings you your data is also wireless, while the truth is that most of the distance is covered by cable-borne data."

This marvel of engineering makes our modern, digital society possible, but it's only in recent years that submarine cables have gone from being general communications pipes to exclusively carrying internet traffic. Back in the late 1980s cables were being laid that could manage a megabit per second, while the latest have a capacity of 24 terabits.

The world's internet traffic is due to rise from five to 14 gigabytes per capita between 2013 and 2018, and that presents a huge challenge. Thankfully, the answer is at the bottom of the ocean.