Who’s Destroying Undersea Internet Cables?

“If a shark bites the internet cable at the bottom of the ocean, how is it repaired? More importantly, how do we even find out where the cable has been damaged in thousands of miles of ocean?

All traffic on YouTube and beyond passes through undersea cables laid on the ocean floor. It’s a data highway we all use daily, but never see.

The undersea cable network is so vast that it connects all the continents. These cables are laid as deep as 8,000 meters, where the pressure is around 5,300 kg per square inch. To imagine it, think of placing the weight of an African elephant on your little toe.

The smallest segment of these undersea cables runs under the Black Sea between Azerbaijan and Turkmenistan, about 300 km long. The largest segment is about 6,600 km and is known as the Marea Cable, which starts from Virginia Beach in the USA and runs through the dark depths of the Atlantic Ocean, ending at Bilbao, Spain.

All the ocean-laid cables in the world add up to a total length of over 1.4 million kilometers. That’s enough cable to wrap around every planet and moon in our solar system—and still have enough left to circle the Earth again!

Laying such long cables at 8,000 meters deep isn’t a simple task. Just laying 1 km of submarine cable can cost up to $2.5 million USD, so you can imagine how expensive 1.4 million kilometers would be.

So why go through all this trouble when satellites can wirelessly connect the world?

Well, there are 493 submarine cables currently active globally. Each cable can handle up to 4,000 terabits per second—that’s enough for 80 million people to stream 4K movies simultaneously on just one cable. In contrast, one satellite can support just 24,000 people streaming 4K at the same time.

To meet growing demand, we’d need to launch a lot more satellites—costly to build, launch, and maintain. If a satellite suffers a major fault, replacing it could cost $400 million USD. That’s why undersea cables are more efficient and cost-effective.

At any moment, billions of gigabytes of data are flowing through these undersea highways. These cables are surprisingly not very thick—about the width of a garden hose, with the actual fiber-optic core inside being as thin as a hair.

This core is made of glass or plastic, through which light travels. This light carries data in tiny packets. First, data from a computer is converted from electrical signals into light signals using a transmitter (typically using lasers or LEDs). These light pulses represent 1s and 0s—presence of light means 1, and absence means 0.

This light travels through the core, which is surrounded by another layer called the cladding. The cladding reflects the light back into the core using Total Internal Reflection, ensuring the signal stays within.

But over long distances, light signals weaken. So, every 100 km, repeaters are installed to boost the signal. To power these repeaters deep in the ocean, where no electricity is available, a DC current of up to 100,000 volts is sent through a copper layer around the cable.

Fiber-optic cables can also transmit multiple signals at once using a method called Wavelength Division Multiplexing (WDM)—different colors of light carry different data streams.

Since light travels at 299,792 km per second, your data travels across the globe in the blink of an eye. When it reaches its destination, a photo-detector converts the light back into electrical signals for your computer or phone to display as graphics or content.

Despite the strength of these cables, they can be damaged by earthquakes, shark attacks, and underwater volcanic eruptions. But the most common cause is fishing.

Some fishing vessels use trawl doors—large metal frames dragged along the ocean floor to catch bottom-dwelling fish. If the trawl door drags over a cable, it can severely damage it.

So how do we locate and repair a damaged cable in such a vast ocean?

Today’s submarine cables are about 1 inch thick, and 1 km of cable weighs around 1,400 kg—roughly the weight of a sedan. Thousands of kilometers of cable are laid in one piece using special ships.

After selecting a location through thorough research, large rolls of cable are loaded onto the ship and laid on the seafloor using a robotic machine that digs about 1 meter deep, places the cable, and covers it back up—very slowly and carefully.

For example, laying the 6,600 km Marea cable between the US and Europe took around 2 years. When one ship’s cable roll ends, another vessel is brought in to continue the job, connecting the new roll to the previous one.

Weather is the biggest challenge—the Atlantic Ocean has strong winds year-round, which can turn into hurricanes. As of now, while researching this video, a Category 1 hurricane is heading for Ireland, forecasted to arrive on August 23rd. Such conditions often pause installation work for days.

Now imagine how hard it must be to repair a damaged cable!

Each submarine cable is connected at both ends to data centers, which monitor the signal 24/7. If the signal breaks, they know the cable is damaged. But pinpointing the exact location requires a special device called an Optical Time Domain Reflectometer (OTDR).

The OTDR sends a light pulse through the cable. When it hits the break, the light reflects back. By measuring how long the reflection takes and the signal strength, they can calculate exactly how many kilometers away the break occurred.

They do this from both ends to verify the fault location. Then, the DC supply is shut off, and ROVs (Remotely Operated Vehicles) are sent to the ocean floor.

The ROVs inspect the cable, pull it up to the surface ship, and the damaged section is cut and replaced with a new one. Since the fiber core is made of glass, the repair must be done with extreme precision so that the glass pieces align perfectly—a process known as splicing.

If the cable is short or the seafloor is too deep, they first lift one end, splice in an extra piece, and lay it back down. Then, they repeat the process on the other end.

Submarine fiber-optic cables are a marvel of modern engineering, and hopefully, this video helped answer your questions about how they work, how they’re installed, and how they’re repaired.

Thank you so much for your kind support and comments.