The layer of the Earth thought to be a solid, iron-rich zone is the inner core.

Here's why:

* Composition: The inner core is primarily composed of iron (around 88%) with a smaller percentage of nickel and trace amounts of other elements.

* Solid State: Despite the immense heat and pressure at the inner core, the iron is in a solid state due to the extreme pressure. Imagine squeezing a sponge so hard that the water inside can't escape, even though it's being heated.

* Location: The inner core lies at the Earth's center, surrounded by the liquid outer core.

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