In the 1930s, seismologists faced a big mystery about the inner structure of the Earth. Today, on what would have been Danish seismologist Inge Lehmann's 127th birthday, it's a good time to appreciate how — using limited data and rudimentary technology — she brilliantly arrived at the answer.

At the time, scientists believed the Earth's core was made of molten rock, surrounded by a solid mantle, then the crust. This model explained why when big earthquakes occurred, certain types of seismic waves weren't detected in particular places on the other side of the world: the waves were bent when they traveled through liquid materials. As the core bent them, it created a "shadow zone" where no waves could be felt.

But after a 1929 earthquake near New Zealand, Lehmann and others noticed something odd: some of these waves could be faintly detected by seismometers in Europe. If the core were entirely molten, this shouldn't have been possible.