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 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 bottom of the glacier came away from a height of 5,200 metres.

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

1,200 metres is higher than the tallest building in the world.

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

On the way it took rock and mud with it, all of it.

When it reached the bottom, it drove into a river in Tibet.

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

That ice and rock blocked the river.

The water could not get past. It stopped above the blockage and piled up.

It was not a real dam. It was a heap of rock that had just fallen down.

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

All of the water that had piled up went down at once.

Below the mountain there were many people.

That area is a famous trekking route, and many people come from other countries to walk it.

So far, 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 are still changing every day.

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

That morning, instruments around the world recorded a magnitude 4.4 earthquake.

So the earliest reports were written this way: there was an earthquake, and then the glacier collapsed.

It sounds reasonable.

Then the USGS changed what it said.

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

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

One corner of a mountain came down heavy enough to move seismometers around the world.

That supposed magnitude 4.4 was later recalculated as a 5.2.

Do you see the order here?

It was not the earthquake that shook the glacier loose.

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

Here, the earthquake is not the cause. It is the result.

How did the scientists work it out?

From satellite photographs.

The first day’s photographs were useless — cloud and dust were in the way.

On the second day the cloud cleared, and they could see it was not only ice that fell. The rock under the ice fell too.

The rock loosened first, and the ice came down with it.

One question still has no answer: why did the rock loosen this time?

Scientists say it is still too early to say directly that this was climate change.

Think about it:

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