He tells her that she will be sent to France as a secret agent, and if she's caught, she's going to be shot.

I'm Helen Obalam Carter, and this is history's secret heroes, where I shine a light on extraordinary stories from World War II.

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We are your weekly guide to the numbers in the news and in life.

And I'm Tim Harford.

As part of the fast-moving argument over US military support to Ukraine,

the US demanded $500 billion worth of access to what was variously reported as Ukraine's rare earths or rare metals or rare minerals. That might all sound like the same thing,

but in fact, the rare earths are a very specific group of elements.

Lutecium, prometheum, samarium and terbium, etrium, etherbium and fulium and urbium,

europium, galilineum, dysprosium, neodymium, cyrium, scantium, home all 17 rare earths, with apologies to the great Tom Lera.

So those are the 17 rare earths, and they're used a lot within traditional industries such as glassmaking and ceramics,

or for making catalysts. They're also used to make very powerful magnets,

which go into electric vehicles and into offshore wind turbines.

But what about the $500 billion? I spoke to Ellie Sacklet Vala,

head of non-ferrous metal pricing at Argus Media, a global commodity price reporting agency.

I started by asking her, are rare earths rare?

They are famously not rare.

Well, the rare earths, those 17 elements that we refer to, they are not technically rare.

They are quite abundant in the earth's crust. I love the fact that you say

...