Hello and welcome to more or less on the BBC World Service, where your weekly guide to

the numbers all around us in the news and in life.

And I'm Tim Harford.

This week, a puzzling claim that the seas around the Chegos Islands are 100 meters below

sea level.

But before that, the Russian invasion of Ukraine has understandably been dominating the

news and listeners have been asking what to make of some of the figures they've been

hearing and reading.

The war was preceded by a build-up of Russian troops on the borders of Ukraine.

But how many troops?

Numbers such as 100,000, 150,000 and even 190,000 have been circulating.

So I started by asking Henry Boyd, who's a research fellow for defence and military analysis

at the International Institute for Strategic Studies in London about those estimates.

In parallel with intelligence estimates being produced in government, open source analysts

have been using commercial, social media and satellite imagery to follow rail movements

inside Russia to the Ukrainian border.

And they've been able to basically track these units from their home bases to staging

areas near the Ukrainian border.

These reinforcements are mostly composed of what are called battalion tactical groups.

In peacetime, Russian units based on Ukraine's border could generate about 28 battalion

tactical groups.

By mid-February, the intelligence estimates suggest this has increased to 120.

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