A. Moisten and filters incoming air
The part of the worm that serves as its brain is the cerebral ganglion
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The mantle is made of a solid rocky substance.
Easy answer: its too hot.
The deepest man made hole is Kola Superdeep Borehole. That was a Russian experiment to see how deep you could drill into earths crust, reaching down 12,262 meters. At that depth the temperature reached 180 °C. The scientists estimated that a 15,000 meters depth the temperature would reach 300 °C, and at that temperature the drill would cease to work and the project was stopped.