How did earthquake waves reveal a solid ball inside Earth's core?

How do you know what is at the centre of the planet when nobody has ever been there, or ever can? The answer is earthquakes. Every large quake sends waves through the whole planet, and a seismograph on the other side records what comes out. If the waves behave strangely, something strange is in the way.

The British geologist Richard Dixon Oldham compared many earthquakes and found a jump in the travel times of the S waves, the sideways kind, at about 120 degrees away from the quake. He read it as a dense core that bends the waves. S waves cannot cross a liquid, so their absence beyond the core suggested that part of it is liquid. In 1913 Beno Gutenberg located the core from seismograms.

Then came the twist. Until 1936, scientists believed the core was a single huge ball of molten metal. A Danish seismologist, Inge Lehmann, looked at data from an earthquake in New Zealand in 1929. Waves of significant strength turned up in Russia, in a place where the theory said there should be a shadow with nothing in it. They had passed through the shadow.

Lehmann realised that the molten core must have something harder in its middle, which reflects and bends the waves. In 1936 she published a paper called P′, that gave Earth three layers below the crust, the mantle, the liquid outer core, and a solid inner core that waves travel through faster.

She had fought for the chance to do it. At Cambridge she faced sex-based barriers and was not allowed to take full part in her studies. She nonetheless became head of the seismological department at the Geodætisk Institut in Denmark.