I'm going to take a different stance for a moment on the term in bold italics above. The most that someone has ever dug into the surface of the earth is 12.262 km, specifically the Kola Superdeep Borehole in the Pechengsky District in Russia.
When they reached the deeper depths, expecting to find molten substances, they only found superheated water (that superheating seeming to have been the result of intense pressure) and hydrogen gas (the latter being a by-product of the water, as the oxygen bonded with earth element). The core samples that were pulled gave evidence of deep sources of water, which serve as physical proof of the account of "the fountains of the deep" which burst forth in the account of Noach in B'resheet 7 and 8.
We've all seen the grade school images in science texts, showing the usual crust/mantle/outer core/inner core model, but there has not been direct, perceivable evidence to show that such a model holds true. Simply put, no one's ever dug down farther than 12.262 km, and, when looking at a world with a diametre of ~12,742 km, that comes up really short. So, how do they know what's down past 12.262 km? The school model sounds like an interesting hypothesis, but nothing's been observed to verify it.
I imagine that is a good thing, given what we read about what comes out of the deep in the Last Days (cf. Chazon (Revelation) 9).
