While the idea that continents have drifted over time dates back to the 16th century, it wasn't until the 20th century that scientists figured out how plate tectonics work.

This geologist made not one but two major contributions to our understanding of how the Earth works. Read about his geologic time scale and his contributions to the theory of continental drift.

Until the late 18th century, most people believed the Earth was about 6,000 years old. Hutton changed this belief by proposing that geologic forces operate at the same rate today as in the past.

Each one of the thousands of earthquakes that occur every year offers a brief glimpse of what's happening deep inside the Earth. Lehmann used seismic signals to change our knowledge of the Earth's core.

Working without the aid of a computer, this mathematician went back 6,000 years and calculated how cycles in the Earth's orbit affect the advance and retreat of the polar ice caps.