From "Manual of Muad'Dib" by the Princess Irulan

I have seen this place in a dream , he thought.
The thought was both reassuring and frustrating. Somewhere ahead of him on this path, the fanatic hordes cut their gory path across the universe in his name. The green and black Atreides banner would become a symbol of terror. Wild legions would charge into battle screaming their war cry: "Muad'Dib!"
It must not be , he thought. I cannot let it happen .
But he could feel the demanding race consciousness within him, his own terrible purpose, and he knew that no small thing could deflect the juggernaut. It was gathering weight and momentum. If he died this instant, the thing would go on through his mother and his unborn sister. Nothing less than the deaths of all the troop gathered here and now—himself and his mother included—could stop the thing.
Paul stared around him, saw the troop spread out in a line. They pressed him forward against a low barrier carved from native rock. Beyond the barrier in the glow of Stilgar's globe, Paul saw an unruffled dark surface of water. It stretched away into shadows—deep and black—the far wall only faintly visible, perhaps a hundred meters away.
Jessica felt the dry pulling of skin on her cheeks and forehead relaxing in the presence of moisture. The water pool was deep; she could sense its deepness, and resisted a desire to dip her hands into it.
A splashing sounded on her left. She looked down the shadowy line of Fremen, saw Stilgar with Paul standing beside him and the watermasters emptying their load into the pool through a flowmeter. The meter was a round gray eye above the pool's rim. She saw its glowing pointer move as the water flowed through it, saw the pointer stop at thirty-three liters, seven and three-thirty-seconds drachms.
Superb accuracy in water measurement , Jessica thought. And she noted that the walls of the meter trough held no trace of moisture after the water's passage. The water flowed off those walls without binding tension. She saw a profound clue to Fremen technology in the simple fact: they were perfectionists.
Jessica worked her way down the barrier to Stilgar's side. Way was made for her with casual courtesy. She noted the withdrawn look in Paul's eyes, but the mystery of this great pool of water dominated her thoughts.
Stilgar looked at her. "There were those among us in need of water," he said, "yet they would come here and not touch this water. Do you know that?"
"I believe it," she said.
He looked at the pool. "We have more than thirty-eight million decaliters here," he said. "Walled off from the little makers, hidden and preserved."
"A treasure trove," she said.
Stilgar lifted the globe to look into her eyes. "It is greater than treasure. We have thousands of such caches. Only a few of us know them all." He cocked his head to one side. The globe cast a yellow-shadowed glow across face and beard. "Hear that?"
They listened.
The dripping of water precipitated from the windtrap filled the room with its presence. Jessica saw that the entire troop was caught up in a rapture of listening. Only Paul seemed to stand remote from it.
To Paul, the sound was like moments ticking away. He could feel time flowing through him, the instants never to be recaptured. He sensed a need for decision, but felt powerless to move.
"It has been calculated with precision," Stilgar whispered. "We know to within a million decaliters how much we need. When we have it, we shall change the face of Arrakis."
A hushed whisper of response lifted from the troop: "Bi-lal kaifa."
"We will trap the dunes beneath grass plantings," Stilgar said, his voice growing stronger. "We will tie the water into the soil with trees and undergrowth."
"Bi-lal kaifa," intoned the troop.
"Each year the polar ice retreats," Stilgar said.
"Bi-lal kaifa," they chanted.
"We shall make a homeworld of Arrakis—with melting lenses at the poles, with lakes in the temperate zones, and only the deep desert for the maker and his spice."
"Bi-lal kaifa."
"And no man ever again shall want for water. It shall be his for dipping from well or pond or lake or canal. It shall run down through the qanats to feed our plants. It shall be there for any man to take. It shall be his for holding out his hand."
"Bi-lal kaifa."
Jessica felt the religious ritual in the words, noted her own instinctively awed response. They're in league with the future , she thought. They have their mountain to climb. This is the scientist's dream . . . and these simple people, these peasants, are filled with it .
Her thoughts turned to Liet-Kynes, the Emperor's planetary ecologist, the man who had gone native—and she wondered at him. This was a dream to capture men's souls, and she could sense the hand of the ecologist in it. This was a dream for which men would die willingly. It was another of the essential ingredients that she felt her son needed; people with a goal. Such people would be easy to imbue with fervor and fanaticism. They could be wielded like a sword to win back Paul's place for him.
"We leave now," Stilgar said, "and wait for the first moon's rising. When Jamis is safely on his way, we will go home."
Whispering their reluctance, the troop fell in behind him, turned back along the water barrier and up the stairs.
And Paul, walking behind Chani, felt that a vital moment had passed him, that he had missed an essential decision and was now caught up in his own myth. He knew he had seen this place before, experienced it in a fragment of prescient dream on faraway Caladan, but details of the place were being filled in now that he had not seen. He felt a new sense of wonder at the limits of his gift. It was as though he rode within the wave of time, sometimes in its trough, sometimes on a crest—and all around him the other waves lifted and fell, revealing and then hiding what they bore on their surface.
Through it all, the wild jihad still loomed ahead of him, the violence and the slaughter. It was like a promontory above the surf.
The troop filed through the last door into the main cavern. The door was sealed. Lights were extinguished, hoods removed from the cavern openings, revealing the night and the stars that had come over the desert.
Jessica moved to the dry lip of the cavern's edge, looked up at the stars. They were sharp and near. She felt the stirring of the troop around her, heard the sound of a baliset being tuned somewhere behind her, and Paul's voice humming the pitch. There was a melancholy in his tone that she did not like.
Chani's voice intruded from the deep cave darkness: "Tell me about the waters of your birthworld, Paul Muad'Dib."
And Paul: "Another time, Chani. I promise."