How would you get by without the internet? You might cope without reading this, but would your bank work? Your energy supplier? Your hospital?
We all get used to the idea of the internet being a digital phenomenon but 97% of the world's data is carried by physical infrastructure, mainly fibre optic cables (the rest goes via satellite).
These cables are almost completely owned by private companies and in the UK's case they all, at some point, lie under the sea. And that's where they are vulnerable.
A quick online search will show you not only where they lie but also the choke points where they come ashore. There are surprisingly few of those.
So the cables could be relatively easily cut. Today, the former Director of GCHQ, Robert Hannigan, told News at Ten the result could be devastating;
"Potentially the economy could grind to a halt, especially financial services," he said.
"If you cut enough of those cables - and there aren't that many landing sites in the UK - but if you cut the key cables you would cease to be able to use the internet.
"We would become a very small, dark island. That's at the worst extreme."
He was responding to a report written for the Policy Exchange think tank by Conservative MP Rishi Sunak who thinks Britain's critical infrastructure is far too vulnerable.
He says "A successful attack would deal a crippling blow to Britain's security and prosperity. The threat is nothing short of existential."
And no one is in any doubt about the principle the source of the threat: Russia. When the Russians invaded Crimea one of their first actions was to cut the data cables.
Russian 'Yantar' class intelligence ships, equipped with submersibles, are already believed to have been operating near cables in the North Sea and the Atlantic.
No wonder Sunak is urging cable operators to invest in sensors along their routes, and the Government to include undersea infrastructure in the next Defence and Security Review.