The Physical Internet: Undersea Cables, ISPs, and What the Cloud Really Is

We've traced a packet's entire logical journey — addressed, named, chopped up, made reliable, and routed across the world. But every one of those steps has been happening on top of something we've quietly ignored: actual, physical stuff. Cables. Buildings. Power.

This chapter pulls back the curtain on the tangible internet — and the first fact is the most surprising one, because the marketing of the last twenty years ("wireless," "the cloud") has hidden it so thoroughly.

The big reveal: the internet is cables

Despite Wi-Fi and 5G and "the cloud," the internet is overwhelmingly a physical, wired thing. Almost all of the world's data travels as pulses of light through fibre-optic cables — hair-thin strands of glass carrying laser light, moving data at a large fraction of the speed of light.

The wireless part is real but tiny: your Wi-Fi and mobile signal cover only the last few metres or kilometres, from your device to the nearest cabled connection point. The instant your data reaches that point — a router, a cell tower's base — it drops onto physical fibre and stays there for essentially its entire journey.

Under the oceans: the cables holding the world together

Here's the fact that reliably astonishes people. Continents are connected not by satellites, but by submarine cables — bundles of fibre, often no thicker than a garden hose, laid across the ocean floor by specialised ships.

Undersea fibre cables

~99%

Satellites

~1%

Roughly 99% of all intercontinental internet traffic runs through a few hundred of these submarine cables. Satellites — even modern low-orbit constellations — carry only a sliver, because fibre is vastly higher-capacity and lower-latency. When you video-call another continent, you're almost certainly talking through a cable on the seabed.

This has real consequences:

• Cables get cut — by ship anchors, earthquakes, fishing trawlers. When a major cable is severed, whole regions can slow down or reroute until repair ships (which can take days to arrive) fix it.
• Geography concentrates them — cables make landfall at a limited number of coastal points, and squeeze through chokepoints like the Red Sea and the straits near Singapore. A few damaged cables in the wrong place can degrade connectivity for entire countries.

The internet, for all its abstraction, has physical chokepoints on the ocean floor.

The ISP hierarchy: how networks stack up

Back on land, the autonomous systems from the routing chapter aren't all equals. Internet Service Providers form a rough hierarchy of tiers:

You

↓→

Local / access ISP

↓→

Regional ISP

↓→

Tier 1 backbone (global reach)

• Tier 3 / access ISPs — the local providers you actually pay, delivering the connection to your home or phone. They buy access to the wider internet from bigger networks.
• Tier 2 / regional ISPs — larger networks spanning regions or countries, connecting access ISPs upward and peering with each other.
• Tier 1 networks — a small club of global backbone providers whose networks are so extensive they can reach the entire internet without paying anyone for access. They form the internet's core, exchanging traffic among themselves as equals. Everyone else, directly or indirectly, connects up to them.

Your packet to another continent typically climbs this ladder — access ISP → regional → Tier 1 backbone → across the ocean → back down the ladder on the far side — the physical embodiment of the BGP hand-offs from the last chapter.

IXPs: where the networks physically meet

For all these networks to exchange traffic, they have to connect somewhere physical. That somewhere is an Internet Exchange Point (IXP) — a building (or set of buildings) where hundreds of different networks run their cables into shared switching equipment and plug directly into one another.

IXPs are the internet's town squares. Their payoff is keeping local traffic local:

This is also where big content providers place servers to sit close to users — the same locality principle behind Content Delivery Networks, which cache content near you so it doesn't have to cross an ocean on every request.

"The cloud" is just buildings

Which brings us to the phrase that has done the most to hide the physical internet: "the cloud." It sounds weightless, ethereal, everywhere and nowhere.

It is none of those things. The cloud is data centres — vast warehouses packed floor-to-ceiling with tens of thousands of physical computers (servers), consuming enormous amounts of electricity, cooled by industrial systems, run by companies like Amazon, Google, and Microsoft.

When you "store a photo in the cloud," you are copying it onto a specific hard drive in a specific building in a specific place on Earth. When an app "runs in the cloud," it's running on rented computers in one of these warehouses. The cloud didn't dematerialise computing — it centralised it into giant facilities you rent space in instead of owning the hardware yourself.

Recap

• The internet is overwhelmingly physical and wired — data travels as light through fibre-optic cable; Wi-Fi and 5G cover only the final few metres.
• Continents are linked by a few hundred undersea fibre cables carrying ~99% of intercontinental traffic — not satellites — with real chokepoints and real vulnerability to cuts.
• ISPs form a tiered hierarchy, from local access providers up to a handful of global Tier 1 backbones that reach the whole internet without paying anyone.
• Internet Exchange Points (IXPs) are physical buildings where networks plug directly into each other, keeping local traffic local — faster, cheaper, more resilient.
• "The cloud" is data centres — real warehouses of real computers drawing real power; storing something "in the cloud" means putting it on a specific machine in a specific building.

We've covered what the internet is, logically and physically. One question remains: with no central router and no owner, who actually runs this thing? Continue to Who runs the internet.