In 1942 the geophysicist Keith Bullen published a paper describing the earth's internal layering in which he used letters to designate various layers from the surface to the core. In this scheme, the earth's lower mantle was known as the D layer.
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The layer below the earth's outer layer is mesosphere. It is the 3rd layer.
Crust
The crust.
No. the layer that actually reaches the surface is the troposphere.
The mantle.