MAMMOTH PRISMS AND spindly spires stretched across the pale blue sky, catching the sun in a ripple of rainbow shimmers. Stairlike forms seemed to stretch forever, crossing and weaving in an endless dance of structure and light. From a distance, the crystalline fingers and branches looked fragile, like coral growing in a fish tank. But at close range, the giant prisms were as solid as marble columns, and as smooth and dazzling as diamonds.

Dwarfed by the towering crystals were five young humanoids; they soared among the spires like birds flying through a forest. Sails billowed from their arms and legs to catch the convection currents, but the fliers mostly depended upon graceful acrobatics to propel themselves. Tandra, the team leader of the five youths, glided to a thick green monolith, tagged up like a swimmer making a lap turn, then bounded off in an altered direction.

Two young Elaysians followed her lead, bouncing off the same crystal and racing after her. The other two members of the science team took different angles at the big monolith and soared off in parallel trajectories. One of the boys rolled into a midair somersault, just having fun. This close to the core of the planet, crystals grew in profusion, so there were plenty of smooth surfaces for pushing off.

Tandra frowned, deepening the crease around her V-shaped forehead ridges. Once they reached the hollow core, she knew they would have to be more careful with their flight patterns. She glanced over her shoulder to make sure the robotic hover-platform was following them. At a discreet distance, the meter-long disc floated along, its small thrusters making minute course changes. Once they reached the core, Tandra knew they might need the platform, for a push-off or a roost.

Laughing and chatting, the five students soared from one spidery, crystalline structure to another, plunging deeper into the center of the unique planet. To the few visitors who came here, Gemworld looked more like the intricate skeleton of a planet, or a giant snowflake made of prisms. For the Elaysians, it was a crystalline aviary. For the other sentient races, Gemworld was what they made of it. All of the inhabitants knew it was a special place, even if very few of them had ever seen a conventional planet for comparison.

As Tandra flew through a stand of rainbow-hued prisms -- old growth before the natural geometry had been improved by fractal models -- she gaped at the exquisite beauty. She thought angrily about the outsiders who claimed that Gemworld wasn't a real planet. They pointed to its forcefields and lack of mass, thinking that such a place had to be artificial, despite its immense size. They simply didn't understand.

Although the crystals looked cold and foreboding, even with their uncanny beauty, they sheltered a surprising variety of life. What could an outsider know about that? Outsiders couldn't stay long enough to really appreciate Gemworld because the low gravity was harmful to most gravity-dependent species. Although humans' muscles were a dozen times more developed, they could never soar between the sparkling archways like Elaysians born and raised in this gossamer cage.

A clearing loomed on the far side of a thicket of crystals, and Tandra realized they were getting close to the core of Gemworld. At one time, it had been a molten, metallic mass like a conventional planetary core, but the Ancients had sacrificed it to fuel the crystal. Now the inhabitants were slowly rebuilding the core with new crystal growth, which was the reason for the students' outing today.

Despite the fact it was empty -- or because of it -- Tandra found the core of Gemworld an eerie place to travel. Since it was hollow and equidistant from the massive crystal constructs surrounding it, the core was the most weightless part of the planet. Only trace gravity was measurable here.

As they flew farther into the ancient heart of the planet and the old-growth crystal, the light became more refracted and eerie, as if the ancient ocean were still there. Tandra felt the weight of ages, how many ages nobody knew, except perhaps the Lipuls. And even they had gaps in their history. The sparkling hues of the upper levels had given way to rust, olive, and sage colors, and the weathered facets were striated from elements that hadn't assaulted them for millions of years.

Tandra could well imagine the great oceans that pummeled these prisms eons ago, when even the Lipuls and the Gendlii were single-celled animals. Gemworld had been young then. Now it was old. But it was still a planet, thought Tandra, despite its low gravity.

Glancing around, she noted that her friends gave little regard to their solemn surroundings as they soared and tumbled through a swooping archway. They were just a flock of young people on a field trip, thought Tandra, and she shouldn't judge them too harshly. Hovering so close to the birthplace of the planet always gave her a sense of history -- and a chill -- but today it seemed more eerie than usual.

"We'll hold up at the purple prism!" Tandra shouted to her comrades.

"Oh, we can make it across!" scoffed one of the boys, soaring past her.

"We'll hold up!" she shouted back. "I'm the team leader."

Tandra knew she couldn't do much if the others disobeyed, but none of them wanted to get stuck out here in the wilderness. They wanted to measure the new stand, see if the fractal modeling program was working as expected, and get out of here. It was too lovely a day to be plodding through kilometer after kilometer of empty air. Although the outing had been fun so far, it had also been long, and the young Elaysians were impatient to return to their research base about four hundred prisms above them.

One by one, the students alighted softly on the ancient purple spire at the edge of the clearing. Tandra gazed into the hollow marrow of the crystal and could see its pulsing gel center. It was a sight she always found reassuring, because it meant that Lipuls probably lived within the marrow. Before the students lay a wide expanse of nothingness, broken up by little islands of discarded equipment, dust, and debris, all of it floating uselessly. In the distance, the edge of the crystal growth shimmered like a mirage.

"How much farther?" asked Lucio, the cutest of the boys, with his slight frame and rare dark hair.

"I'll find out." From her backpack, Tandra took a small handheld device and turned it on. She waited a few seconds until the positioning device communicated with the shell that circled the planet and fixed their current position. As the students hovered around her, Tandra punched in the coordinates of their destination.

"About two hundred prisms." Tandra took a bit of ground crystal from her pocket and tossed it into the air to check the air currents. Now she knew what kind of arc to take to stay on course. "We can cover it in an hour if we get good jumps. Everyone, take a drink, because there won't be time to stop."

Taking her advice, the young Elaysians pulled out sip tubes and sipped from water bladders built into their backpacks. One by one, they used the hovercraft to crouch on the rough-hewn monolith then bound off into the wide-open wilderness. Strung out like a line of migrant birds, the Elaysians soared gracefully across the hollow core of Gemworld.

Tandra realized she would have to rely heavily on the positioning device, so she took it out of her backpack and slipped the strap around her neck, letting it float behind her. A quick glance assured her that the rest of the team and the hover-platform were following dutifully behind, then she double-checked to make sure they were on course. Finally satisfied that they would reach their destination in due time, the team leader spread her sails to catch the prevailing air currents.

It didn't take long for her to realize that something was wrong. Clumps of old mining equipment, which she used for landmarks, never materialized. Tandra checked the device hanging from her neck and discovered that they were considerably off course. That was odd. Tandra had always been the best flier in her class, as demonstrated by her promotion to team leader. Flying on course came naturally to her, but not today. Not down here in the bowels of the planet.

The dimness was more disturbing than usual, as if something were cutting off what little sunshine filtered down to the core. It was almost as dark as twilight, which was the closest they ever came to having a night on Gemworld. The air smelled dry, chalky, hotter than usual. Tandra felt a nervous prickling on her white skin, and her triangular forehead ridges deepened.

"Lucio!" she called. "We're going to stop, and I want you to check the tricorder."

"Why stop?" he responded. "We're making great time!"

"We're off course," she answered. "I want to know why."

A laugh came from Honroj, who was flying closely behind Lucio. "Could it be that our team leader is fallible?"

"Could be," she answered. "I hope it's that simple."

Lenora, who looked enough like Tandra to be her sister, caught a gust in her sails and swooped over Tandra's head. "If we stop," she complained, "we'll never get up to this pace again."

Several of the others shouted in agreement, but Tandra had her mind made up. And she wasn't in a mood to argue.

"Aid me," she ordered the hover-platform. Thrusters kicked on, and the small disc-shaped drone cruised to her side, letting her grab ahold of its low handles. She worked the control panel and stopped the platform, also stopping herself. The other students cruised ahead for some distance, but they soon unfurled backsails and began to float. Tandra noted how long it took for them to slow down, and she also saw something in the distance, something that shouldn't have been there.

She fumbled in her backpack for a pair of lenses, which she pulled over her eyes. At once, the empty distances evaporated, and she could see the apparition ahead of them. It appeared to be a cluster of crystals, where none should be, growing in unruly profusion. Even more disturbing was the color of the new growth: dark and cancerous.

It has to be a mistake, thought Tandra, a trick of the dim light. Things looked funny down here, where distance couldn't be judged by the ubiquitous crystals. Gripping the hover-platform, the young Elaysian put the conveyance into forward and cruised toward her friends. "Link up!" she shouted. "We're checking something out!"

"What?" came one response.

"Says who?" asked another.

She ignored the questions and kept cruising toward her friends, letting the platform do all the work. In a maneuver which they had performed since infancy, the Elaysians drifted together and linked hands. They were still grumbling about the unplanned stop when Tandra snagged Lenora's hand and towed the others behind her like a chain of paper dolls.

When she was certain the link was solid, Tandra increased speed until they were clipping along briskly. She had no spare hand to hold lenses to her eyes, but she could see the cluster of dingy crystals looming ahead of them. It must be huge. The others could see it, too, and they had stopped grousing. In fact, a worried hush had fallen over the team because none of them had ever seen a stand of crystals that looked anything like this one.

"What is it?" breathed Lenora.

"I don't know," admitted Tandra. Normally they would have released hands by now to fly individually, but there was an urgency to their mission. In the face of this unknown threat, it felt safer to hold onto each other.

As they drew closer, Tandra could see the nourishment strands hanging like spiderwebs around the ragged cluster. The strands stretched into the vastness, connected to distant relays. But who would feed this monstrosity?

"Is somebody growing that?" asked Lucio, voicing the thought on everyone's mind. "Is it an experiment?"

"It's not in the logs," answered Tandra. "Nor on any chart I've ever seen."

"Who would claim it?" asked Honroj. No one could answer his question, and the students fell back into an uneasy silence as they soared in unison through the deepening gloom.

With every passing moment, the mass of stunted, deformed crystals loomed larger and more foreboding -- like an icy meteorite with spikes. Tandra almost ordered the group to turn around and flee from the anomaly; but they were training to be scientists, and scientists didn't run from the unknown. They would find a logical explanation for this aberration, she told herself. It had to be a failed experiment or a terrible accident. The young Elaysian didn't want to think about what it really looked like -- a part of Gemworld's heart that was rotting from within.

Tandra cut the power on the hover-platform, and the students glided slowly into the shadows of the jagged cluster. Seen up close, the crystals were even more alarming than seen from a distance; they were twisted, broken, and streaked with imperfections. There were no fractal modeling programs at work here, no careful stimulation and feeding of the crystal -- there were only rampant, obscene growth. Tandra shuddered, thinking that no power on Gemworld was capable of mutating the crystal to such a degree.

As soon as they stopped drifting, Lucio pulled a tricorder from his pack and began to take readings. A look of horror flashed across his handsome face.

"What is it?" she asked.

"Thoron radiation," he answered. "Enough to cause some damage. We've got to get out of here!"

"Not until we've taken a sample of it," said Tandra with determination. From her pack, she removed a small hammer and chisel. Even without being told about the radiation, she could sense danger permeating the dark growth.

Pushing off from the platform, Tandra drifted toward one of the largest, most deformed prisms. It looked like a blackened tree, burned in some monstrous fire. Tandra felt the odd sensation of gravity, and she wondered if these mutant crystals were somehow more dense than the typical variety. Her friends hung back. They obviously wanted to help, but were paralyzed with fear and indecision, and she couldn't really blame them. They needed a cutting for the professors, for the laboratory, Tandra doggedly told herself.

Without giving it a thought, she uncurled her legs to land on a dark facet of the prism. The moment her toes touched, she realized what a terrible mistake that had been -- the murky crystal shattered at contact, and Tandra was engulfed in jagged shards and sooty powder. It burned her skin like an acid bath, and she coughed uncontrollably as she plunged into the crumbling morass. She couldn't stop her momentum! The deformed monolith broke in two, and the upper half closed on top of the young Elaysian like a clamshell.

"Tandra!" shouted Lucio. He and the others started to fly to her aid, but a black cloud spewed outward, forcing all of them back. They closed their eyes and shielded their faces, but even the diluted debris was noxious.

For several seconds, Lucio fell victim to a wracking cough. When he recovered, he realized that he had dropped his tricorder. Braving the venomous debris, he opened his eyes and spotted the device floating only an arm's length away. He quickly snagged the tricorder and checked to see if it was still working.

Lucio heard the others shouting at one another as they crowded around the hover-platform, but he tuned them out. There was only one thing he wanted to see -- the lifesign display on the tricorder. He aimed the device toward the dark cluster and adjusted its field. Trying to ignore the disturbing data he was picking up, he concentrated on the lifesign scan. Tandra has to be alive. She just has to be!

When the first pass was negative, he tried again. And again.

"Lucio!" shouted Honroj, waving to him from the platform. "Link up with us! We've got to rescue Tandra!"

"No, we don't," answered the young Elaysian grimly. "She's dead."

"Shouldn't we look ... for her body?"

"No. The thoron radiation is at dangerous levels. If we don't get out of here now, they'll be looking for all of our bodies."

Clenching his jaw to fight back the tears, Honroj piloted the disc to Lucio's side and grabbed his hand. One by one, he picked up the other students until he had collected a dispirited chain, linked by hands. With worried glances and stifled sobs, the young Elaysians pulled away from the dull, spiny mass of malformed crystals growing at the heart of their world.