尼泊尔和西藏之间的冰川塌了。仪器上那个四点四级的地震，其实是山掉下来的声音。

Get the order straight first, because the order is the part of this story most worth remembering.

Last Wednesday, at 8:40 in the morning, something happened in a valley between Nepal and Tibet.

It happened at a mountain called Langtang, and there is a glacier on it.

That morning, a huge block of ice at the glacier’s lower end came away from the mountain at a height of 5,200 metres.

It fell straight down 1,200 metres before it reached the valley floor.

What does 1,200 metres mean?

It is higher than the tallest building in the world.

A block of ice that size does not come down alone.

On the way down it swept up rock and silt, all of it.

When the whole mass reached the bottom, it drove into a river inside Tibet.

What happened next is the deadliest step in the whole thing.

That ice and rock sealed the river channel.

The water could not get past. It stopped upstream, rising higher and higher, and formed a landslide lake.

To be clear: that was not a real dam.

It was a heap of freshly fallen rubble, full of gaps.

It could not hold for long, and then it broke.

Water that had been piling up for who knows how long went down all at once, in a very short time.

Downstream, there were many people.

That area is a well-known trekking route, and every year large numbers of foreigners come specifically to walk it.

As of now, Nepal has confirmed close to four hundred dead.

Another six hundred trekkers are unaccounted for, from thirty-four countries.

On the Tibet side, more than five hundred people are also missing.

These numbers change every day, and almost only in one direction.

Now the part of this story people most easily get backwards.

That morning, seismic networks around the world all recorded a magnitude 4.4 signal.

So the first wave of reports was written this way: first there was an earthquake, and the earthquake shook the glacier down.

That order sounds entirely reasonable. It matches our instinct about disasters.

Then the United States Geological Survey changed what it said.

They said there had been no earthquake at all that day.

The signal the instruments caught was given off by the landslide itself.

One corner of a mountain coming down was large enough to move seismometers worldwide, all at once.

The signal first read as a 4.4 was later recalculated as a 5.2.

Do you see it? The order here has been inverted.

It was not the earthquake that shook the glacier down.

It was the glacier coming down, doing an impression of an earthquake.

In this event, the earthquake is not the cause. It is the result.

So how did the scientists put the order back the right way round?

From satellite photographs — and from photographs they had to wait a day for.

The images from the day itself were basically useless, blocked solid by cloud and the dust thrown up.

On the second day the cloud cleared, and they could see something more important:

It was not only ice that came down.

The block of rock underneath the ice came down too.

In other words, the ice did not slide away on its own.

The ground holding the ice loosened first, and the ice went down with the ground.

Which pushes the question one layer deeper: why did that rock loosen?

The scientists’ answer for now is restrained.

They say it is still too early to put this particular event down to climate change.

That is a deeply unsatisfying sentence.

But it also reminds us that the only order proven in this story is: the ice first, the earthquake after.

Think about it:

When we watch something happen, why do we so often get the cause and the result the wrong way round?