Tullis Onstott (January 12, 1955 – October 19, 2021) was a professor of geosciences at Princeton University who has done research into endolithic life deep under the Earth's surface. In 2011 he co-discovered Halicephalobus mephisto, a nematode worm living 0.9–3.6 km (0.56–2.24 mi) under the ground, the deepest multicellular organism known to science. He won a LExEN Award for his work "A Window Into the Extreme Environment of Deep Subsurface Microbial Communities: Witwatersrand Deep Microbiology Project". In 2007, Onstott was listed among Time Magazine's 100 most influential people in the world.