The layer of the Earth that is mostly solid metal is the outer core.

Here's why:

* Inner Core: The inner core is a solid ball of mostly iron and nickel. However, it's incredibly hot and under immense pressure, so it's more like a very dense solid.

* Outer Core: This layer is liquid, primarily composed of iron and nickel. While it's technically liquid, it's so hot and dense that it behaves more like a very viscous fluid.

Let me know if you have any other questions!