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

If this fails, that's water wasted , Jessica thought. But it won't matter then, anyway .
With his knife, Paul cut open the power pack, spilled its crystals into the water. They foamed slightly, subsided.
Jessica's eyes caught motion above them. She looked up to see a line of hawks along the rim of the fissure. They perched there staring down at the open water.
Great Mother! she thought. They can sense water even at that distance!
Paul had the cover back on the paracompass, leaving off the reset button which gave a small hole into the liquid. Taking the reworked instrument in one hand, a handful of spice in the other, Paul went back up the fissure, studying the lay of the slope. His robe billowed gently without the sash to hold it. He waded part way up the slope, kicking off sand rivulets, spurts of dust.
Presently, he stopped, pressed a pinch of the spice into the paracompass, shook the instrument case.
Green foam boiled out of the hole where the reset button had been. Paul aimed it at the slope, spread a low dike there, began kicking away the sand beneath it, immobilizing the opened face with more foam.
Jessica moved to a position below him, called out: "May I help?"
"Come up and dig," he said. "We've about three meters to go. It's going to be a near thing." As he spoke, the foam stopped billowing from the instrument.
"Quickly," Paul said. "No telling how long this foam will hold the sand."
Jessica scrambled up beside Paul as he sifted another pinch of spice into the hole, shook the paracompass case. Again, foam boiled from it.
As Paul directed the foam barrier, Jessica dug with her hands, hurling the sand down the slope. "How deep?" she panted.
"About three meters," he said. "And I can only approximate the position. We may have to widen this hole." He moved a step aside, slipping in loose sand. "Slant your digging backward. Don't go straight down."
Jessica obeyed.
Slowly, the hole went down, reaching a level even with the floor of the basin and still no sign of the pack.
Could I have miscalculated? Paul asked himself. I'm the one that panicked originally and caused this mistake. Has that warped my ability?
He looked at the paracompass. Less than two ounces of the acid infusion remained.
Jessica straightened in the hole, rubbed a foam-stained hand across her cheek. Her eyes met Paul's.
"The upper face," Paul said. "Gently, now." He added another pinch of spice to the container, sent the foam boiling around Jessica's hands as she began cutting a vertical face in the upper slant of the hole. On the second pass, her hands encountered something hard. Slowly, she worked out a length of strap with a plastic buckle.
"Don't move any more of it," Paul said and his voice was almost a whisper.
"We're out of foam."
Jessica held the strap in one hand, looked up at him.
Paul threw the empty paracompass down onto the floor of the basin, said: "Give me your other hand. Now listen carefully. I'm going to pull you to the side and downhill. Don't let go of that strap. We won't get much more spill from the top. This slope has stabilized itself. All I'm going to aim for is to keep your head free of the sand. Once that hole's filled, we can dig you out and pull up the pack."
"I understand," she said.
"Ready?"
"Ready." She tensed her fingers on the strap.
With one surge, Paul had her half out of the hole, holding her head up as the foam barrier gave way and sand spilled down. When it had subsided, Jessica remained buried to the waist, her left arm and shoulder still under the sand, her chin protected on a fold of Paul's robe. Her shoulder ached from the strain put on it.
"I still have the strap," she said.
Slowly, Paul worked his hand into the sand beside her, found the strap. "Together," he said. "Steady pressure. We mustn't break it."
More sand spilled down as they worked the pack up. When the strap cleared the surface, Paul stopped, freed his mother from the sand. Together then they pulled the pack downslope and out of its trap.
In a few minutes they stood on the floor of the fissure holding the pack between them.
Paul looked at his mother. Foam stained her face, her robe. Sand was caked to her where the foam had dried. She looked as though she had been a target for balls of wet, green sand.
"You look a mess," he said.
"You're not so pretty yourself," she said.
They started to laugh, then sobered.
"That shouldn't have happened," Paul said. "I was careless."
She shrugged, feeling caked sand fall away from her robe.
"I'll put up the tent," he said. "Better slip off that robe and shake it out." He turned away, taking the pack.
Jessica nodded, suddenly too tired to answer.
"There's anchor holes in the rock," Paul said. "Someone's tented here before."