'Last time I looked, the Scorpion's Unimog was heading north and the quads were heading back this way,' said Hex.

'Into the storm,' said Paulo.

'Yeah, but I'm sure they would've seen it coming and turned back to the village. They'll be sitting this out just like we are.'

Paulo nodded, but his face was worried as he stared out into the storm. 'I wish we could be sure,' he said.

Hex smiled as an idea hit him. 'We can,' he said.

'How?'

'The tracker unit,' said Hex. 'It's in the door pocket next to you.'

Paulo pulled out the slim black box, opened up the lid and extended the telescopic aerial. Hex leaned in as he activated the screen. A grid appeared, with compa.s.s bearings and a distance scale in the top corner.

'That's us,' said Hex, pointing to a bright green double blip in the centre of the grid. 'And that's them,' he added, pointing to another green blip over in the north-west quadrant of the screen. 'See? That's exactly where the village is. Told you they'd be holed up there.'

'So what is that?' asked Paulo, pointing to a third, smaller blip nearly at the top of the screen, heading north.

Hex frowned, then looked back to the blip in the village. He studied it more closely and realized that it was made up of two flashing green lights, not three.

'Did you say the Scorpion was heading north?' asked Paulo.

The same thought hit them simultaneously. They shared a horrified glance, then turned back to the tracker unit. The single blip was nearly off the screen now.

'The Scorpion's got one of them,' said Hex.

'Which one?' asked Paulo, staring at the single blip. 'And how can they still be moving in this?'

'The storm can't have caught them yet,' said Hex.

The blip reached the edge of the screen and winked out. Paulo thumped the dashboard in frustration. 'Quickly! You must call the authorities and give them the Scorpion's position. Maybe they can head him off from the north.'

Hex nodded and reached for his palmtop, but the pouch under his shirt was empty. A chill ran through him as he remembered that he had left his palmtop on the rock just before the storm hit. 'It's out there,' he said quietly. 'On the rock where I was sitting.'

'Inferno!' cursed Paulo. He peered out through the windscreen, trying to judge whether he could make it to the rock without getting lost. He never had the chance. Suddenly the whole vehicle shuddered as something hugely heavy fell on top of them with a ma.s.sive whump. The cab was plunged into darkness and the noise of the storm stopped in mid-howl as though a switch had been flicked.

'What the h.e.l.l. . . ?' Hex ducked as the metal roof of the cab groaned, then folded downwards in a V shape above his head. It came to a stop just above the headrest of the middle seat. The windscreen buckled under the pressure with a crack like a starter pistol and the gla.s.s splintered into a spider web of fine cracks.

Paulo reached for the door handle and yanked it back, but the door would not budge. Hex tried his door with the same result. They stared at one another and their eyes were wild in the dim, green light from the tracker screen. Neither of them could explain what was happening and that scared them.

'Window,' panted Hex. 'Try the window.'

Paulo grabbed the handle and gave it a powerful wrench just as Hex finally realized what had fallen on to the Unimog.

'No!' yelled Hex, but he was too late. Paulo's window slid halfway down and an avalanche of sand poured into the cab. It filled the floor well and reached their knees with frightening speed. By the time Paulo had managed to force the window almost closed again, the sand was up to their hips.

'Dios!' yelled Paulo, holding the tracker unit out of the way of the sand. 'What is happening?'

'It's the dune,' said Hex. 'The base was right up against the side of the wadi. A whole section must've been ready to collapse into it and the storm just pushed it over the edge.'

Paulo swallowed and peered across at Hex in the dim, green light from the tracker screen. 'The dune has fallen into the wadi? Onto us?'

Hex nodded. 'Some of it at least.'

'How much is "some"?' asked Paulo, staring at the solid wall of sand that pressed against the cracked windscreen.

Hex shrugged. 'I don't know,' he said. 'But one thing's for sure. We're buried.'

THIRTEEN.

'I don't understand it.' Amber stared down at her GPS unit, then up at the empty s.p.a.ce in front of the dune. 'This is supposed to be accurate to within fifty metres. They should be here. The wadi should be here and they should be parked in it. But there's nothing!'

Alex pulled his headcloth away from his face and stood up on his quad to get a better view. The sun was only just over the horizon and it was difficult to tell the difference between substance and shadow. A muscle jumped in his jaw as he looked around. He could not quite believe what was happening. Li had been taken away in the Scorpion's Unimog while he and Amber had been pinned down all night in a ferocious sandstorm and now Paulo and Hex had disappeared too. The situation just kept on getting worse.

Alex shook his head, then turned his quad and raced along the edge of the dune base, looking for tyre tracks. He rounded a curve in the side of the dune and a chasm opened up in front of him. With a curse, Alex swung the quad hard to the right, leaning into the turn with all his might. His rear left wheel skidded over the edge of the wadi and the quad started to tip. He found some extra power in the motor and forced the quad away from the edge. He came to a halt well away from the dune and scrambled from the machine.

By the time Amber caught up with him, Alex was standing on the bank at the place where he had nearly lost control of his quad. 'Look at this!' he called to her as she climbed from her machine and hurried over. 'I don't understand it. Here's the wadi at the base of the dune, but it stops dead, right there, as though it ran into a wall.'

'Or the other way round,' said Amber softly, staring wide-eyed at the section of dune they had just skirted.

'What?'

'I think I know where they are, Alex,' said Amber, her voice high with fear.

'Where?' asked Alex.

Amber pointed at the new curve in the dune. 'Under there.'

They raced back to their original position and turned off the quad engines. 'This is it,' said Amber, studying the screen of her GPS unit. 'This is where we left them last night.'

'Listen!' said Alex. 'Did you hear that?'

They both stood stock still, holding their breath. Very faintly, from the slope directly ahead of them, came the sound of a truck horn. They threw themselves at the slope and started digging feverishly, scooping the sand out with their bare hands and sending it arcing back between their legs. The horn sounded again, louder this time, and they threw themselves into the digging with renewed effort.

'Got something,' panted Amber as her hand connected with metal. She brushed the sand away and revealed part of the roof of the Unimog cab. Quickly they found the edge of the roof then cleared the sand away from the side window. It was partly open, but the opening was blocked with sand. Alex knocked the sand away and peered inside. It was still too dark in the cab to see anything but there was no mistaking the sour air that sighed into his face.

'Quick!' he yelled, returning to clearing the sand from the cab door. 'They're out of oxygen!'

They dug until their muscles were screaming, and finally there was enough s.p.a.ce to force the door open. Sand poured out of the cab but there was no other movement in the dim interior. Paulo was slumped to one side, clutching the tracker unit to this chest. His face was grey and there was a blue tinge to his lips. Hex was resting against the steering wheel, his head turned towards them. His eyes were closed.

'Are we too late?' quavered Amber.

Just then, Hex's eyelids fluttered open. His green eyes tried to focus, then rolled back in his head. He reached up to the steering wheel and pressed the horn lever. The horn blared out and Hex gave a small nod of satisfaction before his eyes fluttered shut again. He must have been sending this signal in his semi-conscious state for hours.

'We're here, you idiot,' said Amber in a voice choked with tears. 'Enough of the signalling!'

Together, Amber and Alex dragged Paulo and Hex from the cab. Once they were out in the fresh air, the two boys recovered quickly and soon Paulo was sitting up, still clutching the tracker unit and looking around groggily. Hex stumbled to his feet, leaned over and vomited into the sand.

'You took your time,' he gasped, when he could talk again.

'Yeah, well, we would've found you sooner if you hadn't decided to play go seek,' retorted Amber, handing him the container of water from her quad pannier.

Paulo's eyes suddenly became sharp and focused. He looked down at the tracker device, then up to Amber and Alex. 'Li?' he croaked.

'She's with the Scorpion,' said Alex. 'She's under cover, pretending to be a boy called Liang.'

'And you let her go?' said Paulo softly, staring at Alex.

'Yes,' muttered Alex, gazing down at his boots.

Paulo surged to his feet and aimed a punch at Alex's face. His big fist landed squarely enough but he was still too weak to put any power behind it. Alex fell backwards on to the sand with his cheekbone smarting but still intact. He clambered to his feet again but did not retaliate. Part of him felt he had deserved the punch.

'You let her go?' repeated Paulo, staggering as he prepared to punch Alex again.

'Stop it, you big idiot!' yelled Amber, putting herself between Paulo and Alex. 'It's not Alex's fault. You know what Li's like. Once she gets an idea into her head, no-one can stop her.'

For a few seconds Paulo glared at Alex over Amber's shoulder, then the fire left his eyes and he nodded slowly, acknowledging the truth of what she was saying. He stumbled and would have fallen hard if Alex had not caught him. Gently, Alex lowered Paulo to the sand, where he sat with his head down, cradling the tracker unit to his chest.

'She's wearing her tracker device,' said Alex. 'She wanted us to track her to the Scorpion's headquarters, then bring in the authorities.'

'It was a good plan,' mumbled Paulo.

'Still is,' said Hex briskly. 'They must've been caught in the storm too, so they can't be that far ahead, and we know they're heading north. We can catch up with them.'

While Paulo and Hex recovered from their near-suffocation, Alex and Amber dug around the buried Unimog. There was no sign of Hex's palmtop, which meant they had no way of communicating with the outside world, but they did manage to retrieve two jerry cans of fuel and two girbas of water that had not burst under the weight of the sand. The quads' fuel tanks were nearly full and there were bottles of water in the panniers, but they did not know how long they would be following the slavers: they would be no good at all to Li if they ran out of water or fuel in the middle of nowhere.

As well as water bottles, the quad panniers each held a first aid kit, high-energy emergency food rations, a pair of night-vision goggles and two pairs of pigskin gloves and ski goggles. When Paulo and Hex had recovered enough to travel, they all pulled on a pair of the gloves, wrapped the headcloths tightly around their necks and faces and fitted the ski goggles over their eyes. This was essential wear for daytime quad travel through the desert. The strong sun and drying wind meant they had to be completely covered from head to foot to protect against dehydration; even then they would have to stop every hour to drink.

Alex sat astride one quad, with Hex behind him, holding the tracker unit. Paulo took the other quad and Amber clambered on behind. Paulo had rigged some webbing to carry the extra jerry cans of fuel and Hex and Amber each had a girba hanging at their backs.

'Ready?' called Alex, over the revving of the quad engines.

Amber leaned over Paulo's shoulder to make sure the GPS unit on the quad's handlebars was set, then she gave a thumbs-up signal to Alex and thumped Paulo on the shoulder. 'Let's go get Li!' she yelled.

Paulo set a fast pace and the two quads cut through the desert side by side as the sun rose in the sky. They travelled north all through the morning, only stopping for water breaks or for Amber to input waymarks into her GPS unit. Every time they stopped, Paulo looked over at Hex and Hex shook his head. The tracker screen remained depressingly blank and Paulo's face grew more and more grim.

'Are you sure it is working?' he asked at their fifth water halt.

'Yes,' sighed Hex, trying to remain patient. He pointed to the cl.u.s.ter of four green dots in the centre of the screen. 'It's picking up our tracker signals,' he said, looking up at Paulo. 'That means it's working.'

Paulo's shoulders slumped and he turned away, heading back to his quad. Hex turned back to the screen. He became very still, staring intently at the tracker unit. A small green dot had appeared, right at the top of the screen.

'There she is,' he said softly. 'We've found Li.'

Alex took over the lead, responding to Hex's shouted directions. Li's tracker dot was not moving, so the gap was closing quickly when Paulo suddenly pointed ahead and shouted.

'Tracks! I see tyre tracks!'

They brought the quads to a halt and Alex climbed down to study the tracks. 'They're heading north, just as we thought,' he said, straightening up. 'That's their Unimog all right.'

'Are you sure?' asked Amber.

'I followed these tracks for most of yesterday,' said Alex. 'Of course I'm sure.'

'She's still not moving,' said Hex, staring at the screen.

'They must have stopped for food, or something,' said Amber.

'But where?' muttered Paulo, gazing across the stony, flat plain which stretched ahead of them with barely a dip in the ground. 'According to this, she's less than a kilometre away. We should be able to see the Unimog.'

They looked at one another uneasily.

'There's a small rise ahead look at the rocks over there,' said Amber.

'Yes, but that's not high enough to hide a Unimog,' pointed out Alex.

'It's about a kilometre away,' said Hex, looking from the screen to the rocky rise. 'And it's in the right direction...'

'Come on,' said Paulo, running for his quad.

As they drew closer, Paulo shook his head in puzzlement. The rise was little more than a pimple on the face of the plain. Even the quads would have trouble hiding behind it, but the Unimog tracks were heading straight for it. He looked over to Hex to see whether Li's signal had moved on. Hex shook his head and pointed to the rise.

'They definitely stopped here,' said Alex, as the quads pulled up next to the rise. 'See all the footprints around the tyre tracks?' He picked up a discarded jerry can and sniffed at the open top. 'They refuelled, then carried on north,' he finished, pointing out the tracks heading away from the rise.

'So why is Li's tracker signal still here?' asked Hex, staring at the screen.

'Guys?' called Amber. 'Come look at this.'

As they drew closer to Amber, they could all see the dread on her face. Paulo felt a cold hand trace a path all the way down his back as he saw what Amber was holding out to them. It was a small rock. One side of the rock was smeared with blood and hair.

'Dios,' he breathed, looking around wildly. A few metres away the sand was churned up, as though there had been a struggle there. Paulo hurried over and stopped, staring down at the ground. A strange track led away from the churned-up area. There was a central rut, with deep holes and gouges in the ground alongside it. On the other side, gouts and splashes of something had spilled on to the ground, forming a sticky, brown crust.

Paulo leaned closer and caught the unmistakable smell of blood. Horrified, he pulled back and looked at the others, suddenly realizing what the odd tracks meant. 'Someone dragged themselves away from here,' he said. 'They were bleeding. Lots of blood.'

They all turned and looked up to where the dragging tracks disappeared over the top of the low rise. Amber swallowed and her throat clicked loudly in the silence.

'Li's signal?' she asked.

Hex held out the tracker unit. The dot marking Li's position was still in the centre of the screen, alongside the other four dots. 'She must be over the rise,' he said.

Alpha Force looked at one another, then started up the slope, following the splashes of blood.