Eight states went dark in four minutes. The cause took forty years to grow.

•  A tree grew into a line in Ohio

•  The room that watched it went quiet at 14:14

•  Three more lines opened inside the next hour

•  Eight states and Ontario were out by ten past four

A grid is held together by a margin. Every line runs below its limit so that if one of them opens, the rest can take up what it was carrying. That margin is not a physical object anywhere. It is a calculation, redone continuously, by people watching screens.

On the afternoon of August 14, 2003 the network across the Midwest and Ontario was ordinary. It carried 61,800 megawatts across eight states and one province, serving about fifty million customers on a warm Thursday afternoon. At five past three the investigators later placed the system as electrically secure and able to take the next outage without trouble.

What made it look safe was that nothing unusual was showing. Two units had gone out earlier in the day and neither one mattered on its own. The margin was still there on paper. Nobody in the room had been told that the paper had stopped being written.

Every row below comes from one document, the final report of the joint task force set up by the two governments, published in April 2004. Its investigators rebuilt that afternoon second by second from recorders, exchange logs and the telephone tapes of every operator involved.

Look at 14:14. That is the minute the monitoring system at the Ohio operator stopped working, and nothing announced the fact to the people sitting in front of it. For the next hour and three quarters, every judgement made in that room rested on a screen that had stopped moving. The room was not ignoring the readings. It was reading them carefully, and they were an hour old.

Four things decided how this ended, and the weather was not one of them.

•  The margin was a calculation, not a thing. Investigators placed the system at five past three as secure and able to take the next outage. Nothing physical changed in the hour that followed. What changed was how much anyone knew.

•  The monitoring system stopped and said nothing. It went quiet at 14:14 and stayed quiet for an hour and three quarters. A screen that has stopped updating looks exactly like a screen on which nothing is happening.

•  The trees were the common thread. The task force put inadequate management of growth under the wires as the shared reason three separate high-voltage spans went out that afternoon. One condition, three outages, in one county.

•  Nobody upstream could see it either. The regional coordinator had no clear picture of which lines in Ohio had opened. The report that followed carried forty-six recommendations, and the earliest of them are about knowing the state of the system in front of them.

Eight states and Ontario went out together. In parts of the United States it took four days to bring everything back, and parts of Ontario were on a rota for more than a week after that. Nothing had been physically destroyed, which is the part people forget.

Canada measured what those days did to it. National output for August came in 0.7 percent below where it would have been, and 18.9 million working hours went unworked. The American side was counted in ranges rather than in single figures, which is itself a measure of how wide the reach was.

The rules changed afterwards. Standards that had been voluntary became requirements, and the same network is now published hour by hour in public, which is a direct descendant of that afternoon.

•  A margin nobody can see is not a margin. Every buffer is reported by something. When the reporting stops, the buffer is either still there or already gone, and the room cannot tell which.

•  Quiet is the hardest reading to interpret. An instrument showing nothing and an instrument showing no trouble look identical. Ask what any screen does at the moment it breaks.

•  Ask what separate things rest on together. Three independent spans went out that afternoon for one underlying reason. Independence on a diagram is not independence on the ground.

•  Tight coupling turns small into large. A tree and a quiet printer between them reached eight states and a province in roughly four minutes.

I have sat at a desk where the feed froze and the room carried on quoting off it for nine minutes before anyone looked up.

Ask of anything relied on daily what it looks like when its reporting has stopped, and how anyone would know - a question with an easy answer when the thing in question is an ounce of gold that can be picked up.