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

What has the worm to do with the spice, melange? he asked himself. And he remembered Liet-Kynes betraying a veiled reference to some association between worm and spice.
"Barrrroooom!"
It was like a peal of dry thunder coming from far off to their right.
Again: "Barrrroooom!"
The worm drew back onto the sand, lay there momentarily, its crystal teeth weaving moonflashes.
"Lump! Lump! Lump! Lump!"
Another thumper! Paul thought.
Again it sounded off to their right.
A shudder passed through the worm. It drew farther away into the sand. Only a mounded upper curve remained like half a bell mouth, the curve of a tunnel rearing above the dunes.
Sand rasped.
The creature sank farther, retreating, turning. It became a mound of cresting sand that curved away through a saddle in the dunes.
Paul stepped out of the crack, watched the sand wave recede across the waste toward the new thumper summons.
Jessica followed, listening: "Lump . . . lump . . . lump . . . lump . . . lump . . ."
Presently the sound stopped.
Paul found the tube into his stillsuit, sipped at the reclaimed water.
Jessica focused on his action, but her mind felt blank with fatigue and the aftermath of terror. "Has it gone for sure?" she whispered.
"Somebody called it," Paul said. "Fremen."
She felt herself recovering. "It was so big!"
"Not as big as the one that got our 'thopter."
"Are you sure it was Fremen?"
"They used a thumper."
"Why would they help us?"
"Maybe they weren't helping us. Maybe they were just calling a worm."
"Why?"
An answer lay poised at the edge of his awareness, but refused to come. He had a vision in his mind of something to do with the telescoping barbed sticks in their packs—the "maker hooks."
"Why would they call a worm?" Jessica asked.
A breath of fear touched his mind, and he forced himself to turn away from his mother, to look up the cliff. "We'd better find a way up there before daylight." He pointed. "Those poles we passed—there are more of them."
She looked, following the line of his hand, saw the poles—wind-scratched markers—made out the shadow of a narrow ledge that twisted into a crevasse high above them.
"They mark a way up the cliff," Paul said. He settled his shoulders into the pack, crossed to the foot of the ledge and began the climb upward.
Jessica waited a moment, resting, restoring her strength; then she followed.
Up they climbed, following the guide poles until the ledge dwindled to a narrow lip at the mouth of a dark crevasse.
Paul tipped his head to peer into the shadowed place. He could feel the precarious hold his feet had on the slender ledge, but forced himself to slow caution. He saw only darkness within the crevasse. It stretched away upward, open to the stars at the top. His ears searched, found only sounds he could expect—a tiny spill of sand, an insect brrr , the patter of a small running creature. He tested the darkness in the crevasse with one foot, found rock beneath a gritting surface. Slowly, he inched around the corner, signaled for his mother to follow. He grasped a loose edge of her robe, helped her around.
They looked upward at starlight framed by two rock lips. Paul saw his mother beside him as a cloudy gray movement. "If we could only risk a light," he whispered.
"We have other senses than eyes," she said.
Paul slid a foot forward, shifted his weight, and probed with the other foot, met an obstruction. He lifted his foot, found a step, pulled himself up onto it. He reached back, felt his mother's arm, tugged at her robe for her to follow.
Another step.
"It goes on up to the top, I think," he whispered.
Shallow and even steps , Jessica thought. Man-carved beyond a doubt .
She followed the shadowy movement of Paul's progress, feeling out the steps. Rock walls narrowed until her shoulders almost brushed them. The steps ended in a slitted defile about twenty meters long, its floor level, and this opened onto a shallow, moonlit basin.
Paul stepped out into the rim of the basin, whispered: "What a beautiful place."
Jessica could only stare in silent agreement from her position a step behind him.
In spite of weariness, the irritation of recaths and nose plugs and the confinement of the stillsuit, in spite of fear and the aching desire for rest, this basin's beauty filled her senses, forcing her to stop and admire it.
"Like a fairyland," Paul whispered.
Jessica nodded.
Spreading away in front of her stretched desert growth—bushes, cacti, tiny clumps of leaves—all trembling in the moonlight. The ringwalls were dark to her left, moonfrosted on her right.
"This must be a Fremen place," Paul said.
"There would have to be people for this many plants to survive," she agreed. She uncapped the tube to her stillsuit's catchpockets, sipped at it. Warm, faintly acrid wetness slipped down her throat. She marked how it refreshed her. The tube's cap grated against flakes of sand as she replaced it.
Movement caught Paul's attention—to his right and down on the basin floor curving out beneath them. He stared down through smoke bushes and weeds into a wedged slab sand-surface of moonlight inhabited by an up-hop, jump, pop-hop of tiny motion.
"Mice!" he hissed.