Other
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...
Nine Planets
The Eight Planets: Earth
Excellent Eight Planets site that provides a vast amount of information about planet Earth. Very comprehensive and complete site.
National Geographic
National Geographic: Encyclopedia: Mantle
An in-depth look at the structure and composition of Earth's mantle, with lots of visuals. Covers lithosphere, Mohorovicic discontinuity, asthenosphere, transition zone, lower mantle, the d double-prime region, mantle convection, mantle...
Other
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.