Seismologists think the inner core of our planet is solid, while the outer one is liquid and molten. Next is the mantle upon which the crust is gliding. However, we still have no idea what it consists of because we’ve never reached it: its depth is from 30 to 2,900 km, and the deepest well mankind has dug so far is the Kola borehole in Russia, which is just 12.3 km deep.

Fact: scienceline, quora