Marie Tharp

Photo: The Granger Collection

Marie Tharp (1920-2006)
Field: Oceanographic cartography

Big Deal: A keen mapmaker, Tharp worked to plot the ocean floor, still uncharted territory in the first half of the 20th century. By the late 1950s, her relief maps revealed a massive rift in the mid-Atlantic ridge—a wave-making discovery that helped finally give credence to the theory of continental drift.
Nettie Stevens

Photo: Science Photo Library

Nettie Stevens (1861-1912)
Field: Genetics

Big Deal: In 1905, Stevens published a radical paper arguing that an animal's sex was determined by chromosomal combinations rather than environmental factors, as some believed. How'd she figure it out? By scrutinizing chromosomes extracted from mealworms, termites and other insects.
Grace Hopper

Photo: Bettman/Corbis

Grace Hopper (1906-1992)
Field: Computer science

Big Deal: Decades before Silicon Valley, Hopper, a math professor, created the first computer program to automatically translate typed instructions into machine-readable code. And the computer term bug? Hopper and her team at Harvard popularized it after finding an actual moth in an early prototype.