The layer of the Earth made up of solid nickel and iron is the inner core.

Here's a breakdown:

* Inner core: This is the Earth's innermost layer. It is a solid sphere of mostly iron with a smaller amount of nickel. The immense pressure at the center of the Earth is so great that it keeps the iron and nickel in a solid state, even though temperatures are extremely high (around 5,200° Celsius).

* Outer core: Surrounding the inner core is the outer core, which is liquid and composed of mostly iron with some nickel.

Let me know if you have any other questions about Earth's structure!