Traffic between continents does not travel through the sky in any meaningful proportion. It travels through cables lying on the seabed, and that has been true since long before the internet existed.
Distance is why fibre wins
A signal to a satellite in high orbit and back must cross tens of thousands of kilometres, and the round trip adds delay that no engineering can remove.
A cable crossing an ocean covers a fraction of that distance, so the delay is far lower, which matters enormously for anything interactive.
Capacity compounds the advantage, since a single fibre pair carries an enormous volume of traffic and a cable contains several such pairs.
The cable is mostly protection
The fibres at the centre of a submarine cable are thin, and nearly all of the diameter is steel wire, copper and insulation wrapped around them.
Near shore, where fishing gear and anchors are the main hazard, cables are armoured heavily and often buried in the seabed by a plough towed behind the ship.
In deep water the armour thins out, because the risk of physical contact falls and the weight of a heavily armoured cable becomes the greater problem.
Amplifiers keep the light alive
Light weakens as it travels through glass, so the signal has to be boosted at regular intervals along the route.
Repeaters spliced into the cable do this, powered by a current sent along a copper conductor from the landing stations at each end.
Because a repeater cannot be maintained on the seabed, it is engineered to run for decades without intervention, which is what makes cable laying so expensive.
Breaks are routine and repairs are slow
Cables are cut regularly, most often by anchors and trawling gear in shallow water, and occasionally by seabed landslides.
Repair requires a specialised ship to locate the fault, grapple the cable from the seabed, lift both ends and splice in a new section.
That work takes days or weeks depending on weather and how far the vessel must sail, which is why routes are built with alternatives rather than relying on a single path.
Landing points concentrate the risk
Cables come ashore at a limited number of suitable places, determined by seabed conditions, permits and the location of existing infrastructure.
That clustering means several independent cables often share a narrow corridor, so one incident in the wrong place can affect multiple systems at once.
Regions served by few cables feel any interruption acutely, which is why new routes to less connected areas change local internet performance far more than added capacity on busy corridors.