Much of what we know about the Earth and its four distinct layers can be attributed to a man who was never formally educated in geology. In fact, Francis Birch earned his engineering degree at Harvard, spending two years at an electrical company before dabbling in geophysics with a fellowship studying magnetic properties of matter.

Many of Birch’s studies concerned elastic properties of materials, a measurement for how much something – in this case, elements of the planet’s interior – will compress under pressure. In a 57-page landmark article, published in 1952, Francis Birch described his key findings.

But first, he wrote a joke:

“Unwary readers should take warning that ordinary language undergoes modification to a high-pressure form when applied to the interior of the Earth.” He continued the quip, listing examples of terms such as “vague suggestion” and “perhaps” – arguing they transform into the exaggerative words “positive proof” and “undoubtedly” when pressure is applied.