Dramatic volcanic eruptions occurred in the Spring of 2021 in Iceland and St. Vincent. This column explores the use of a numerical model to understand the giant displacements of the Earth's surface that result from such volcanic activity. The model used was development by Japanese geophysicist Kiyoo Mogi to explain a much older eruption, the 1914 eruption of Sakurajima volcano, located in Kyushu, Japan. Mogi's model was so successful, and is still widely used today, because he took a step-by-step approach to solving this complicated problem, making simplifying assumptions where he could, and using data to the maximum extent possible to estimate a reasonable solution.