The answer goes back to the collision of plates whereby oceanic plates (i.e. the Pacific, Nazca, Juan de Fuca, Australian-Indian, & etc.) collides with the continental plates of the continents.

But since oceanic plates are heavier, they then subducts [goes underneath] the continental plates thereby "uplifting" the continents & thus uplifts the mountain ranges.

Inshort, what lies several kilometers underneath the highest mountain ranges of the world are layers of continental & oceanic plates!

However, some of the oceanic plates that got subducted underneath the continents have nowhere to go but back down into the depths of the earth a.k.a. "the inferno."

*It's not the hell that we know where Satan & his demons reside but the earth's inner core made up of Iron.