How does the internet actually cross oceans?
Through submarine fibre optic cables lying on the seabed — not satellites, which carry a negligible share of intercontinental traffic despite what most people assume.
The scale. Well over 95% of international data travels on undersea cables. There are several hundred in service, totalling well over a million kilometres.
What a cable is. Surprisingly thin — roughly the diameter of a garden hose in deep water, thicker where armoured near shore. Inside are a small number of optical fibre pairs, each carrying many wavelengths of light simultaneously through wavelength division multiplexing, surrounded by steel wire strength members, a copper conductor and waterproof insulation.
Repeaters every 50–100km amplify the optical signal, powered by a high-voltage current sent along the copper conductor from shore — thousands of volts across thousands of kilometres.
Where they land. At landing stations, a small number of coastal sites where cables come ashore and connect to terrestrial networks. These are concentration points and are regarded as critical infrastructure.
Who owns them. Historically consortia of telecommunications operators. Increasingly, large technology companies build and own cables outright, because they move enormous volumes between their own data centres and it is cheaper than buying capacity.
How they break. Roughly a hundred and fifty to two hundred faults a year, the large majority from human activity — fishing trawlers and ship anchors in shallow water. Others come from undersea landslides, earthquakes and abrasion. Deliberate damage is rare but has become a prominent security concern.
Repair requires a specialist cable ship to locate the fault, grapple the cable from the seabed, lift it, splice in a replacement section and lay it back. It takes days to weeks depending on weather, depth and ship availability, and there are relatively few such ships worldwide.
Why single failures rarely matter. Traffic reroutes automatically over other cables within seconds. Outages become visible only where a country or region has few cables — several nations have been substantially disconnected by one or two cuts.
Why satellites are not the answer for bulk traffic. Geostationary orbit imposes a round-trip latency of over half a second by geometry alone. Low-earth-orbit constellations reduce that considerably and are transformative for hard-to-reach places, but their total capacity remains a small fraction of what cables carry.