Satellites are the visible symbol of global communication, and the overwhelming majority of international internet traffic travels through cables on the ocean floor. The reason is a comparison satellites cannot win on the terms that matter.
Fibre carries vastly more data
A single optical fibre transmits many separate wavelengths of light simultaneously, and a submarine cable contains multiple fibre pairs.
The result is capacity measured in enormous quantities per cable, and that capacity can often be increased by upgrading the equipment at each end rather than laying new cable.
Radio spectrum available to satellite systems is finite and shared, which caps how much can be delivered over a given area.
Distance to orbit costs time
A signal to a satellite in high orbit and back travels a long way, and the resulting delay is noticeable in conversation, gaming and any exchange requiring rapid back-and-forth.
Light in glass covers an ocean far faster than a signal can make a round trip to distant orbit, which is why financial and cloud infrastructure depends on cable routes.
Constellations in low orbit reduce this delay substantially, which is why they have changed the comparison for consumer access without displacing cables for bulk transport.
The cables themselves are unremarkable objects
In deep water a submarine cable is roughly the diameter of a garden hose, with fibres at its centre protected by steel wire and a copper conductor that powers repeaters along the route.
Repeaters amplify the signal at intervals so it can cross thousands of miles, and they must operate for decades without maintenance.
Near shore the cable is armoured heavily and often buried, because that is where damage occurs.
Breaks are common and mostly accidental
Cables are cut regularly, and the usual causes are fishing gear and ship anchors in shallow water, along with undersea landslides and seismic activity.
Repair requires a specialised vessel to locate the fault, lift the cable, splice it and return it to the seabed, which takes days or weeks depending on distance and weather.
Traffic reroutes automatically over other cables, so most breaks are invisible to users unless a region depends on very few routes.
Ownership shifted toward the platforms
Cables were historically built by consortia of telecommunications carriers sharing the cost.
Large technology companies now fund cables directly, because their own traffic between data centres justifies the investment and gives them control over routing and timing.
That shift has increased how many cables are built, and it has concentrated a share of the world's connectivity in the hands of a small number of private owners.