What is 4000 km below Earth's surface?

Why group 1 element poor complexing agents? What is the oxidation state of Pb in Pb3O4? How does composting help global warming? Does bromine have a lower boiling point than chlorine? Is an EF5 tornado worse than a category 5 hurricane? Why do ships seem to disappear over the horizon from the bottom upwards? Why only sodium metal is used for sodium fusion test? Is the mass number an average of the masses of all naturally occurring isotopes of an element? Why building a home on an alluvial fan might not be a good idea? What type of rain falls in the south east? Which is not a common cause of soil mismanagement? How a quadrat could be used? Rocks that have large grains you can easily see are described as fine grained true or false? What of these does not cause physical weathering? Is Erratic erosion or depositional? How does gravity relate to a streams ability to erode and deposit? Why smoking is dangerous while taking a walk in a forest in very hot and dry weather? What stream velocity would be required to carry the smallest boulders? How does global warming affect the marine biome? How do you think corals would be affected by cloudymuddy water?

What is 4000 km below Earth's surface?