In 1798, from his measurements of gravitational attraction, Lord Cavendish, illustrated at left, reported the density of Earth to be 5.48 times the density of water, a result quite close to 5.53, the modern measured value [1].

One hundred years later, Emil Wiechert, pictured at right, realized that the density of the Earth is greater than the density of rock, meaning that the Earth cannot be composed entirely of rock.

In 1898, Wiechert suggested that the Earth might be like a giant meteorite with a core of nickel-iron metal which had settled to the center like iron settles from slag on the hearth of an iron mill. Wiechert justified his idea with calculations showing that the Earth’s density could be explained if the Earth has a core of nickel-iron metal (like the iron meteorites he had seen in museums) surrounded by a shell, or mantle, of rock [2].

Less than a decade elapsed before the English seismologist, Richard Dixon Oldham, pictured at left, discovered the Earth's core [3]. In 1906, while investigating the speed of earthquake waves as a function of their depth within the Earth, Oldham found that the speed of earthquake waves becomes faster with greater depth of penetration into the Earth, but only to a certain depth. Below that depth, earthquake waves suddenly travel much slower, indicating a profoundly different material. Richard Oldham had discovered that the Earth does in fact have a core, just as Emil Wiechert had envisioned might be the case.

By the early 1930s, seismologists had established the size of the Earth's core and had shown that it is liquid, because it is unable to transmit shear waves.

As 1936 approached, the interior of Earth seemed rather simple, like the representation at right, consisting of a fluid core surrounded by a solid rock mantle, and topped by a very, very thin crust. But then, a mystery was discovered. Earthquake waves were found where it was thought they should not be found. Inge Lehmann solved that mystery and discovered an object within the fluid core, the inner core [4].

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