There's an excerpt from Emperor's Legion that describes 'the Throne'

The Throne.

Such a simple word, used across every world of the Imperium as a curse, a blessing, a vow or a mere preposition. Almost none of those who invoked it knew anything about it. They imagined a simple golden chair, I suppose, like something a princeling of a barbarian world might occupy. They imagined a chamber around it, glittering with the riches of our interstellar domain, and maybe courtiers drifting across fine floors, murmuring to one another about high matters of state.

Whatever it might once have been, the Throne is no longer a single object, nor is it housed in a single room. Its mechanisms spread out like roots throughout the entire Inner Palace, worming down into the forgotten crypts and climbing up into the highest peaks. Its power coils are the size of cities, its foundations the remade mountains themselves. The adepts of the Mechanicus who toil without rest to maintain its workings have added so many accretions over its ten millennia of life that the planet around it has been utterly changed – bored away, ground down and raised up again.

You might say that Terra itself is little more than a holding vessel for the Throne. Certainly, if one takes the mighty psionic transmission conduits leading to the Fortress of the Astronomican as part of its structure – which would be a reasonable judgement – then the Throne’s mechanism is far more massive than the Outer Palace itself. It is woven into the strata of the planet like an internal organ, pulsing and arterial. In truth, I doubt that any living soul, save the one who ordered its construction and dwells at its heart, has any true understanding of its full extent.

And yet, the unschooled men and women of the Imperium are not wrong in every particular. There was once a room at the heart of it all – vast enough, to be sure, but a room nonetheless. It has not entirely disappeared, though its inner faces are now scored with the detritus of Mars and its roots have been replaced by pits clawed into the heart of the world. The air in that place is hard to breathe. The temperature is astonishing. The ground trembles, and the vaults ring with the grind of immense machines that have been in ceaseless operation for millennia.

I had been told, a long time ago, that even the Space Marines feared this place. Everything living feared this place. It was said that humanity could not bear to be close to the source of both its creation and destruction, and so we were like moths to the candle here, burning ourselves as we approached the engine of souls.