Final answer:
The third layer of nature is commonly known to be the mantle, which lies between the Earth's crust and core, and is the largest chemical layer of Earth in volume.
Step-by-step explanation:
The third layer of nature often refers to the layers of the Earth's interior. Specifically, the third layer can denote the mantle, which is the layer lying between the Earth's crust and its core. As described in your reference information, this mantle is distinct from the other layers in that it has an ultramafic composition, meaning it contains more iron and magnesium, less aluminum, and somewhat less silicon compared to the crust. It is also the largest in terms of volume among Earth’s three chemical layers and extends to about 2,900 km in thickness.