To generate steam, on a grand scale, required both a ready supply of water and a very substantial heat source.
After the disappointment of the mud-brick ziggurats the Sumerian lords needed to experiment with boosting and controlling the heat of the sacred fire in the stronger stone pyramids. So they removed from Luxor to a safe distance about 1200 km south to the great bend in the Nile in Nubia (now Sudan), to the remote sandstone monolith of Jebel Barkal where they drove tunnels into the solid rock as a test-bed to determine the limits of manifesting and containing the mysterious energy of the sacred fire.
Much later, around 1500 BC, Jebel Barkal became the site of the temple of Amun-Ra, the official Egyptian sun god, marking the birthplace of the god. It was the place where the Nubians and Egyptians agreed [the energy of] the sun was first manifested on Earth.
The background to that story is buried deep in the muddle of Egyptian mythology. The Egyptians, like the Greeks after them, created confusion by characterising their gods with humanised forms and names. A pantheon of names which changed with the place and time and which were allotted to various characters who were perceived to be sons, brothers or fathers of one another.
So Ra was simply the sun, the primordial sun god; only later personified in the official religion as Amun-Ra. The later humanised figure of Amun-Ra became an earthly personification of Ra and the ‘father’ of Osiris, later Horus – or some say vice versa.
Osiris/Horus was the son of Ra, manifested on Earth. His eye was the sun. He was the original son of god sent down to Earth and made manifest to save mankind from the darkness. Oddly enough, Osiris/Horus was also viewed as the god of ‘transformation’ [into energy], and of ‘resurrection’ of the material body into ‘divine union’ with the essence of Ra.
The difficulty in understanding Ra is that his form and his emanations were invisible. Just as later his humanised form, Amun-Ra, was believed to be ‘hidden’ – as in unseen, as in not visible. Likewise much, much later the Hebrew god YHWH was believed to be without form or substance, ineffable and unknowable; and even later again all that imagery was transferred to the obtuse Christian notion of the ‘Holy Spirit’.
Curiously that old lore of the invisible god was understood in a different way amongst ‘uneducated’ country people. When European explorers first approached the pyramids they found native guides reluctant to go near for fear of illahat ‘evil spirits’ and they scoffed at them for believing in ‘ghosts’. But that was a European misunderstanding – what the guides were afraid of was the memory [or reality] of the unseen emanations of the gods.
In the Middle Kingdom, a thousand years after the construction of the Great Lights, the man-Pharaohs and their high-priests had forgotten all the ancient technology – which they didn’t understand anyway. They preferred to believe the humanised mumbo-jumbo and built the magnificent temple complex of Karnak at Thebes (Luxor) as a memorial to the supposed birthplace of Amun-Ra and his son(s). Karnak became the official birthplace of the manifestation of the sun on Earth.
But the problem with all that was that the Nubian Kings of Kush to the south always stoutly claimed that the son of the sun was born at Jebel Barkal, and they had evidence. Around 1500 BC Pharaoh Thutmose I took his High-Priest of Amun and went to see for himself the state of the ‘true’ religion in Nubia. They were apparently convinced of the Nubian claims – leading his successor Thutmose III to build a great temple at the foot of the ‘sacred mountain’ and officially declaring it to be the southern home and ‘original birthplace’ of Amun-Ra, calling it Ipetsut (the equivalent of Karnak). 
Scholarly analyses are quite firm that the temple at Jebel Barkal, and the nearby garrison town of Napata, was not just established for political expediency but the Egyptians actually believed it really was the source of Amun-Ra. They called the temple ‘Throne of the Two Lands’, preceding Karnak as the original home of the god. The rock itself was called Dju-wa’ab (Pure Mountain) and identified as the font of what they struggled to express as the ‘mysterious primeval aspect’ of the god, Amun-Ra. It was the origin of the spirit or essence of the god – in a material, technological sense.
Curiously the original temple was built tunnelled into the rock, and later religious art showed illustrations of Pharaoh Ramses II making ritual offerings to the figure ‘Amun of Karnak’ – seated inside the mountain.  It was as though the ‘spirit’ of Amun-Ra arose from within the rock.
What did the Egyptians see at Jebel Barkal that convinced them it was the birthplace of the sun on earth?
Jebel Barkal is an isolated sandstone rock outcrop about 100 meters high, with an unusually flat top and an impressive sheer cliff-face on the southern side. The original imposing temple buildings have long since fallen to ruins.
From afar the rock outcrop is very prominent on the flat landscape, but otherwise fairly unremarkable. The cliff-face is ordinary weathered reddish-brown sandstone and the bottom perimeter is littered with fallen debris. It is only the top that appears unusual. Where the sides appear mainly ordinary reddish-brown sandstone the very top layer is blackened and charred as if it has been baked. The top is anomalously and artificially blackened as if burnt.
An aerial view, in full colour, is marvellously presented in the National Geographic magazine, November 1990.  From above the flattened, blackened top is revealed as a massive scar with a clear fracture line evident along the western side, like a crater rim. The remaining rock, within the ‘crater’ scar, is uniformly blackened as if charred by extreme heat; and the boundary rim, especially to the north is strangely sharp as if marking the edge of a melt zone.
It looks exactly as if the sandstone monolith has been exploded, burned and partially melted by a massive force from within. It looks for all the world like it has suffered a mild nuclear explosion and meltdown.
The Sumerian lords had experimented with the design of their nuclear device by conducting controlled ‘ignitions’ within the rock. Until they found the upper limit – the level at which the device exploded and melted solid rock.
For the first time the ‘spirit’ of Ra, the nuclear energy of the sun, was manifested on Earth; an offspring of the sun – the son of god – was manifested on Earth to save mankind from the darkness.
And so a legend was born.
It should not come as a complete surprise that the ‘sacred fire’ of the Sumerians and their Egyptian cousins was some sort of nuclear energy device. Why else would Homer have Aeneas go to so much trouble to rescue the eternal flame of the fire of the hearth? It could not have been just any old ordinary flame. It was ‘sacred’ as in sacrosanct – meaning apart, isolated and untouchable. In ancient Egyptian, the words for ‘sacred’ and ‘isolated’ (or ‘apart’) were the same. The old god was so dangerous that it had to be kept apart in a sacred stone naos in the inner temple – in the sanctum sanctorum, the most isolated holy of holies, accessible only to the high priests wearing safety clothing.
But over time the literal understanding faded and descended into faith. Sacred became holy, hidden, impenetrable, inscrutable, ineffable, unknowable, unutterable, mysterious; without form or face. Myth became magic; spirit became spiritual; reality descended into religion.
The ancient Egyptians – at least the monogene lords – knew the reality. Their texts recorded plainly that Ra was not a humanised sun-god in European Christian terms, but was the primordial ‘essence’ of the sun … the oldest and firstborn son of matter. I came into being from unformed matter.
Its manifestation was awesome but also fearsome; its names were ‘Limitless’ and ‘Dreadful of Fire’, like a blinding light it spread unseen its terror over the Nile … unquenchable, irresistible, slaughtering. 
So the pyramids must also have been isolated – and they were. They were all located along the west bank of the Nile on the ‘away’ side of the dominant winds, and individually guarded by stone walls up to 10 metres high. They were forbidding and forbidden. Not sacred in any religious sense but taboo territory; no-go zones.
All three pyramids of the Giza complex, called ‘Light Land’, were additionally protected by a massive 10 m. outer perimeter wall (now destroyed), called the ‘Wall of the Crow’; and the whole was watched over by the looming red-painted figure of the giant Sphinx, whose Arab name was Aboulhol or Abou Chawl – meaning ‘Father of Terror’. It was a scary and prohibited zone.
The outer perimeter was called ‘Wall of the Crow’ because that was how close a crow would or could fly or suffer death since birds are particularly sensitive to nuclear radiation. When Chernobyl suffered meltdown in 1986 it was reported that birds in the area fell out of the sky. That was why the Book of Jasher recorded that when Abraham caused a ‘fire in the temple’ in Ur all the ravens in the vicinity were killed. The later Greeks called such places ‘Avernus’, an area of ‘black mists’ or vapours so evil no birds would go there.
Perhaps that’s why even into modern times the pyramids were strangely sterile – the only living things encountered by early explorers were bats, which are oddly insensitive to nuclear radiation. And it’s why the local peasants remained strangely fearful of the illahat ‘evil spirits’ – they didn’t suffer religious reverence but rather secular fear of invisible radiation. Or at least the folklore memory of it.
Of course, exactly how the Egyptian lords managed nuclear energy poses many questions – the answers to which remain to be revealed in the secrets of the stars inscribed in the Pyramid Texts.
 Wilkinson, The Rise & fall of Ancient Egypt
 Kendall, http://www.jebelbarkal.org/; http://www.museum.com/ja/showdia/id=2885
 Kendall, Sudan’s Kingdom of Kush, Nat Geographic, 178 (3), Nov 1990
 Book of the Dead; Pyramid Texts