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The National Academies of Sciences, Engineering, and Medicine: Project Mohole
Project Mohole was "the earth sciences' answer to the space program," a project to gain information about the Earth by drilling a hole through the Earth's crust to the Mohorovicic Discontinuity (Moho). This website gives a history of the...
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Geology.com: Mohorovicic Discontinuity the Moho
Explains what the Mohorovicic Discontinuity is, how it was discovered, how deep it is, and why it has not been studied much.