Since the first undersea cable was completed in 1858 to deliver telegraph messages between the US and England, we’ve had an extensive network of undersea cable networks that enable communications between continents.

Earlier this year there were 378 undersea fiber cables in place that stretch over 745,000 miles. Here’s an interactive map that shows all of the cables and also allows highlighting of individual cables.

What’s most intriguing about the map is that there are a few cities around the world where numerous cables terminate. One such place is New York City, and Superstorm Sandy cut the connections of several fibers, and the connection between the US and Europe went dark for a few hours. The companies building the cables are now considering diversifying the terminal locations of the fiber cables.

Cables also routinely get cut from other events such as earthquakes, underwater mudslides, ship anchors, and even a tiny number from sharks. There is an average of about 25 underseas fiber cuts per year. Repairs are made by ships that pull the cut ends of the fiber to the surface and splice the ends back together. There have been a few fiber cuts where there was suspicion of sabotage, but it’s never been proven. There is no real way to provide security for undersea cables, and the routes of the cables are well known. It’s been a poorly kept secret that spy agencies around the world tap into various cables to monitor traffic.

Undersea fibers are made differently than other fiber. Since the biggest danger from fiber cuts is in shallow water, the cable for shallow locations is as thick as a coke can and is routinely buried under the surface. At deeper depths below 8,000 feet, where the dangers of fiber cuts are minimal, the cables are only as thick as a magic marker. There are cables laid as deep as 25,000 feet below the surface. One unusual aspect of underseas fibers is the use of an underlying copper layer that is used to transmit the electricity needed to power fiber repeaters along the long underseas paths. The cables can be powered with as much as 10,000 volts to force the power along the longest Pacific routes.

The undersea fiber paths carry over 99% of the traffic between continents, with the small remainder carried by satellites. Satellites are never expected to carry more than a tiny fraction of the traffic due to the gigantic, and constantly growing volume of worldwide data traffic. The FCC estimated that only 0.37% of the US international data traffic is carried by satellite. The capacity of the newer cables is mind-boggling—the Marea cable that was completed between Spain and Virginia in 2018 has a capacity of 208 terabits per second. No satellite network is ever going to be able to carry more than a tiny fraction of that kind of capacity. Worldwide bandwidth usage is exploding as the users on the Internet continues to grow (there were 1 million new users added to the Web every day in 2018). And just like in the US, usage per person is growing everywhere at an exponential rate.

One thing that is obvious from the fiber map is there are parts of the world or routes that don’t exist. The companies that fund the cables build them to satisfy existing broadband needs, which is why there are so many cables between places like the US and Europe or between countries in the Mediterranean. There are no routes between places like Australia and South America because there is not enough specific traffic between the two places to justify the cost of a new cable route. While cable routes terminate to India and China, one would expect to see more fibers added in coming years. These two countries are currently seeing the biggest number of new Internet users (in 2018 there were 100 million new users in India and 80 million new users in China).

The cables have traditionally been built and owned by the world’s biggest telecom companies. But in recent years, companies like Google, Facebook, Microsoft, and Amazon have been investing in new undersea fibers. This will allow them to carry their own traffic between continents in the same way they are also now carrying terrestrial traffic.

Undersea cables are designed for a 25-year life, and so cables are regularly being retired and replaced. Many cables aren’t reaching the 25-year life because the built-in repeaters become obsolete and it’s often more profitable to lay a larger capacity newer cable.