Where did all the rubble come from?

Is the rubble that makes up the tumulus at the top of Mt. Nemrud nothing more than what it took to level the top of the mountain and then pile up the dome? We have no record of what the mountain looked like before construction began. A few miles east and slightly north, near Lake Van, there is a second Mt. Nemrud that is an active volcano, last erupted in the mid 1600s. It is over 9,500 feet tall. It has a flat top with a caldera lake in it. What if the Throne of the Gods Mt. Nemrud was also fairly flat, even a caldera of some couple of hundred feet in depth. A great pyramid was built in it, actually excavated another two or three hundred feet deep. As the sloping sides of the pyramid were built, the rubble was piled up outside. The volume of the tumulus is equal to the volume of the rock removed to build the pyramid and to level the rim of the caldera.

I suppose it would be a pretty unbelivable feat of engineering to build a pyramid underground. It would be an easier digging feat to invert the pyramid, but that would necessitate building a roof that would support the tumulus. On the other hand, combining the two concepts might work. You dig down to an incredible depth and build the inverted pyramid, then build the regular pyramid on top of it.

Okay, so the biggest problem with this is that you have to quarry the entire excavation first and then build inside it. Why? You'd have to store allt he blocks of stone and then lower them back into the hole tobuild the structure, back-filling as you go.

There is really no reason that the project would need to take any less effort than the building of the pyramids and sphinx of Egypt. In fact, it could leave even less trace of local inhabitation than the pyramids due to its remoteness. The labor could have come from any number of sources for slaves.

In ~36 BCE, Antiochus was strong enough that he could repel the Roman army led by Marc Anthony. A hundred years later, in 72 CE, the Romans marched through Komagene with scarcely any resistance, and did not even know about the Throne of the Gods. Suppose the difference was the state of the concealment of the resting place of the library, designed by Ptolemy I. The move would have been executed by Ptolemy VIII, father of Cleopatra before Caesar & Pompei came to Egypt. Antiochus negotiated with Pompei, avoided Caesar, and defeated Marc Anthony.

The next problem, whatever the engineering feat, is that there has to be a continuing opening or access to the structure underground that enables the use of the library after the top of the mountain was sealed. Somehow the location has to have been disclosed to Gutenberg in order for him to have concealed it in the map and in the printers' marks. I'm thinking that the access point is several miles away, near the Euphrates. They need water and access to food.

So, some thousands of laborers over a period of one to three centuries, excavate a mountain, quarry the building stone, build an underground pyramid, backfill it with rubble and bury a king on top of it so that should it ever be excavated the burial champer would be found and no further excavation attempted, all without being noticed or investigated by anyone who survived to tell about it. And for two thousand years since then, a cult has lived in the structure, preserving the documents and guarding the mountain against discovery. -- Easy!