All hell had broken loose.

Within the mere minutes following Gordon's entry into the test chamber, Eli's worst fears were suddenly confirmed.

It had begun with the shouting from the observation deck, separated though it was by a glass sliding door locked by a retinal scanner. Eli and Isaac had been listening anxiously for word from doctors Guthrie or Backman, to get an impression of how things were going with Gordon in the test chamber. At first, all they heard were the expected instructions given to Freeman: the starting of the rotors, the powering of the stage one and two emitters.

Though nothing of note had happened thus far, Eli had been listening with bated breath. He had been praying inwardly that things would continue to be so predictable. He could make out Backman's tentative voice:

'Uh, it's probably not a problem – probably – but I'm showing a small discrepancy in…well, no. It's well within acceptable bounds.'

Eli cursed his colleague for his brashness. The experiment should have been aborted for even the slightest discrepancy. Didn't they know what the worst possible outcome could be? It just wasn't worth the risk.

Guthrie's self-assured voice was now telling Gordon to turn his attention to the sample:

'…look to the delivery system for your specimen…'

This was the climax of the test. Eli couldn't hear or see Gordon pushing the trolley weighted with exotic matter, but he was picturing it in his mind. The seconds ticked by as slowly as cold molasses. If anything fateful were to happen today, it would happen now.

He could tell immediately from his colleague's shrill voices that something terrible had occurred. The anti-mass had made contact with the phase array, and already there had been a reaction, and an unexpected one.

'Attempting shut down!', shrieked Backman. 'It's not…it's not…it's not shutting down! It's not shutting—'

There was a crack and screams coming from the scientists in the observation deck. Then Eli could hear no more from Guthrie and Backman. The sound of his colleague's voices was replaced, however, by the sound of whatever reaction was taking place in the test chamber. Eli was certain by this point about what was happening.

A Resonance Cascade. The worst possible outcome of this experiment. There was nothing else it could be.

If something unfortunate had happened to those in the observation deck, then something even worse must have happened to the man in the test chamber itself. Gordon Freeman had been new here, recommended to Dr Breen by Kleiner. He didn't deserve to die within his first month at the facility. It should have been somebody with less potential. Somebody older. He hoped Gordon's death had been quick and painless.

Eli could now hear two deafening sounds at once: the ringing of an alarm, the emergency alarm that had never had cause to ring before, as well as a strange sound he could not place. A quick burst which occurred all of a sudden, like lightning.

Eli's first instinct was to reach for the telephone on the wall to call for help, but he found upon attempting to dial a number that the thing was completely dead. Besides, he knew that whatever effects the Cascade would have would not be localised down here. Scientists on the surface would also have their hands full with the reaction. Nonetheless, he and Kleiner would need their help.

The lights had dimmed, computers were sparking and glass was broken.

'Why didn't they listen?' Eli wondered aloud. 'I—I tried to warn them.'

Looking slowly to his left, Eli could see that Kleiner had sunk to the ground in despair.

'I never thought I'd see a Resonance Cascade', murmured his friend. 'Let alone create one.'

Just then, Isaac looked up in surprise, seeming to see someone in the direction of the elevator.

'By Schrodinger's Cat', he exclaimed. 'He's alive!'

Eli turned to see who his colleague was looking at, and was shocked almost beyond words.

'Gordon!' gasped Eli. 'Thank God for that hazard suit.' The young scientist stood before him, completely, and miraculously, unharmed. Eli realised immediately that Gordon was their best hope of escaping the depths of Anomalous Materials.

'All the phones are out', he informed him. 'I can't reach anyone in the facility. You need to get to the surface and let someone know that we're stranded down here. There's no telling what kind of danger—'

Eli was cut off by a yell from Dr Kleiner, and a flash of green from behind. Everyone's attention turned to the cylindrical glass cases against the far wall. Eli was astounded by what he saw inside one of them.

'My God. What have we done?' he uttered.

Isaac Kleiner's reaction was quite different, and he was the first to approach.

'Fascinating.'

Behind the glass was a small quadruped, about the size of a pumpkin, with extended forelegs and a fleshy exterior. It shuffled from side to side restlessly within its confined glass case. Dr Kleiner bent forward to get a closer look.

'Would you look at that.'

Bizarre as the creature may have been, examining it was not the highest priority at that moment, and Eli knew it. He spoke to Gordon.

'We can't afford to waste time', he said. 'You'll need me for the retinal scanner. I'm certain the rest of the science team will help you along the way.'

He lead Freeman to the retinal scanner, who followed him in his habitual silence.

'And for God's sake', continued Eli, 'keep that suit on until you're in the clear. You hear me?'

Gordon inclined his head in assent. Eli pressed his eyes to the scanner, and the door slid open. He watched Gordon leave with nothing but the hazard suit for protection. He couldn't have known then that he would not see the young scientist again for many years.

Eli turned back to Kleiner, who was still examining the creature in the jar where ordinarily there would be a fluorescent portal. All three portals behind their glass cases had now disappeared.

'What on Earth could this creature be, Eli?' whispered Isaac.

'I get the feeling that this thing is not from Earth at all', Eli replied.

'You're saying this creature is from another world?'

'That's what I'm afraid of', uttered Eli. 'You know what a Resonance Cascade entails as well as I do, Izzy. I'm sure that that crystal was not of this world either. The spectrometer caused it to resonate with enough force to open a dimensional rift between Earth and wherever the crystal came from.'

'So the crystal is from another world too…'

'I think so. I'm sure our surviving colleagues are treating this as a disaster – a freak accident. But I'm getting a terrible feeling that this was no accident at all.'

'Then who brought the crystal here?' pondered Kleiner.

'I'm sure Dr Breen would know, but I'm guessing it was brought here by the man we have been haunting Anomalous Materials ever since this anti-mass experiment business started. You've seen him?'

Kleiner nodded. 'You mean the man with the briefcase? In the blue suit?'

'Yes. I once heard him talking to Breen, but couldn't get close enough to hear his words. The cadence of his voice was strange. At times he seemed to make short gasps for air, and he pronounced certain sounds as if English was not a language he was used to speaking.'

'Oh, Eli', chided Kleiner. 'A strange way of speaking isn't enough to confirm that the man is from another world.'

'Maybe not, Izzy, but there was one other time I heard the voice. Very recently.'

'When?'

'He spoke to me just a few moments after Gordon stepped out of this room. He whispered "Prepare for Unforeseen Circumstances" in my ear.

Dr Kleiner was wringing his hands with worry. 'But I was right next to you, Eli. I didn't hear anything. Perhaps you imagined the words.'

'Perhaps', Eli said doubtfully.

Eli and Isaac waited in the data processing room for close to an hour before they started to wonder if waiting was the most sensible thing to do. During this time the alarm had been ringing, but now, all of a sudden, it stopped. The two scientists had heard nothing from the observation deck since the moment the resonance cascade hit. How many of their colleagues were still alive?

Since Gordon Freeman was the only one to emerge from the test chamber, it seemed likely that the other scientists that had let him into the chamber had met their end. That HEV suit had truly been a lifesaver, Eli thought.

Kleiner spent much of this time examining the creature in the glass case, speculating about the function of its elongated limbs (did it use them to jump?), and its underside which appeared to be somewhat hollow, a cavity about as large as a man's head.

Could this be the only life-form to have crossed the dimensional gap between Earth and wherever the sample crystal had come from? If somebody had purposefully sent the anti-matter to Anomalous Materials then surely there was some grander scheme afoot. It seemed possible that something more intelligent could be immigrating from the other world to this one.

The thought of it made Eli's blood run cold. Today's disaster might have repercussions not just for Black Mesa but for the whole world and all the human beings in it.

Remaining in the data processing room at first seemed the logical thing to do, but Eli found that the longer he and Kleiner waited, the likelier it seemed that no one would be coming to rescue them down here, at least for a good few hours. Remaining in the depths of Black Mesa might turn out to be dangerous, especially if his suspicions about the other world and its inhabitants turned out to be true.

If Black Mesa wasn't safe, then his family were just as threatened as he and Kleiner were. As far as Eli was concerned, that made finding Azian and Alyx the highest priority.

'I've made up my mind, Izzy', stated Eli.

Kleiner looked up from the glass case. He had been rapt, scrutinising the alien life-form.

Eli continued: 'If there is one alien creature which has jumped the gap between our world and wherever it is it came from, there are certainly more. Waiting around here is futile. We are like rabbits at the bottom of a rabbit hole down here. At least on the surface we have somewhere to escape to, while down here we are as good as trapped. Besides, everyone else further up is probably evacuating.'

Kleiner did not look hopeful.

'Perhaps there are other life forms', he said. 'Possibly even dangerous ones. But this is a Resonance Cascade. The situation is catastrophic. Dr Breen must have realised how dire things are and have already called in the military. If we wait here they will surely cleanse the upper levels of whatever foreign entities there may be and work their way down to us. The safest option is still to remain where we are.'

Eli saw the sense in Kleiner's words, but could not bring himself to hole up at the lowest point of the research facility like some frightened animal.

'You may be right, Izzy, and if I had only myself to worry about I might agree with you. But I have a wife and daughter to think of. I can't linger down here while they might be in danger. I'm leaving now. Are you coming with me?'

Several emotions cast their shadows over Kleiner's face, one after another: defiance that he was right in thinking that remaining where they were was safest, fear that circumstances may be hazardous on the upper levels, and an even greater fear of being left alone so deep underground with the prospect of more menacing aliens teleporting in. In the end, Kleiner's shoulders sagged in concession.

'Alright, Eli. I'll go with you.'

Seeing his friend's distress, Eli tried to comfort him: 'I'm sure Gordon has cleared the way for us somewhat. If there are hostile aliens ahead then he would have dealt with them with whatever came to hand or lured them away. Besides, our security guards carry pistols. Black Mesa's personnel aren't completely defenceless, you know.'

Kleiner was only partly reassured. Eli returned to the retinal scanner and pressed his face to it. With a beep, the sliding door moved aside. Isaac glanced back one last time at the strange creature behind the glass, then followed his colleague.

Peering into the observation deck, Eli could see that Dr Guthrie was dead. His corpse was sprawled on the floor, surrounded by blood and viscera. His torso seemed to have been ripped open by some sort of beam from the test chamber which had blasted through the broad rectangular aperture. Eli had never seen a dead body before and forced himself to look away. There was no sign of the other scientists.

On peering into the room, Kleiner let out a distressed 'Oh my', looking for all the world as if he was about to be sick, but he swallowed and dogged the footsteps of Eli.

They hadn't taken two steps into the deck when a beam of energy darted from the test chamber window to the far corner of the observation deck. It ripped a hole in the wall panel with a crack and caused the glass in the opposite sliding door to break. It made Eli and Isaac jump back. Clearly, the Cascade hadn't expended all the energy generated from the crystal. It seemed whatever reaction Gordon had survived hadn't ended.

Now, Kleiner wanted more than ever to stay put, but Eli was resolute, so he stuck close to his colleague. Eli recognised a pattern in the beam of energy. There were a couple of seconds in between each discharge, so, crouching low, Eli waited for the moment the beam gave out, then ran to where the opposite door had been, which by now was completely destroyed. He felt a surge of heat hit his back as the discharge tore a fissure into the wall once more.

Looking back, Eli could see Kleiner cowering against the opposite wall. His face was pale with queasiness.

'Come on, Izzy', shouted Eli over the sound of the fracturing panel. 'Just do what I did. Run over here as soon as the beam gives out.'

Keeping as low to the floor as possible, Isaac ran forward as soon as the discharge vanished, cleared the middle of the room, then pressed himself against the opposite wall. He stayed there for some seconds, breathing hard and slowly sinking to his knees. Kleiner clearly had done nothing so dangerous in his entire life.

Doctors Vance and Kleiner made their way through Anomalous Materials with the greatest of caution. Not far from the observation deck, they approached a room which had been torn apart by the Cascade.

Kleiner had wanted to observe one of the crab-like aliens more closely at first, but here they found one that was free of the glass case which had confined the first one. The one in this chamber used its long front legs to launch itself at the pair. They had to duck sharply to avoid its hollowed underside which seemed intent on coupling with the heads of either of them. If Gordon had confronted the little monster, he had evidently avoided it and pressed on, being unarmed. They did the same.

The parade of obstacles continued. In one hallway, a laser had been freed from its conduit and was darting across the room, severing control panels and data processing units. Eli and Isaac passed under it. The door to the next room had malfunctioned and would not open, but one of its lower glass panels had been smashed. Evidently, Gordon had shattered his way through with whatever weapon had come to hand.

Further on, they came to the elevator which had taken them down towards the test chamber's level that morning, just a few hours previously. Somehow, Eli felt that was much longer than that.

Dr Kleiner pressed the button, but the elevator did not appear, so both scientists crouched through the sliding door's broken panel once more and looked down. The lift had crashed at the very bottom of the shaft. Eli prayed that Gordon, or any scientist for that matter, had not been in the car when it did, though he got the feeling that there probably had been.

Eli thought of his family to give himself courage while climbing the ladder upwards towards the higher level. It didn't matter how terrible the horrors he would face today, Azian and Alyx needed him. He had to be strong for them.

The alarms had long since stopped, and now and then Eli and Isaac would come upon a corridor which looked the same as it did earlier that morning, but things kept appearing which reminded him that things weren't the same.

The two physicists now came upon something stranger than the two crab-like aliens they had encountered before. Splayed out on the ground, supine, was a humanoid figure. As they approached, they could see that it was wearing a scientist's lab coat. The familiar characteristics ended there. For a head it had a crab-alien merged with its skull, though the creature was dead this time. The humanoid's arms and fingers were wiry and elongated, ending in sharp, claw-like points, and its torso bore a large, wide-open gash through which Eli and Kleiner could see all its insides.

'Thank God it's dead', sighed Isaac with a shudder.

The walls and floor near the creature were splattered with greenish-yellow blood, no doubt from the alien and not the previously human body.

Whoever had killed this monster, thought Eli, must have had a reason to. And with such long, deadly nails it wouldn't be far fetched to surmise that it had been behaving aggressively before it died. Either Gordon or a security guard must have dispatched it.

'But now we know what the crab-creature's hollow underside is for', mused Eli. 'We know what happens when it couples with a human's head. It somehow fuses itself with the host's brain and takes over their motor functions. That's my theory anyway.'

'Based on what we've seen, I'd say that's a likely hypothesis', said Kleiner. His tone became thoughtful. 'But I wonder how aggressive they'd be if their harmful parts were removed. It might be possible to keep one as a pet.'

'Don't be ridiculous Izzy. These monsters are dangerous, and more than that, disgusting.'

Eli ended his examination of the corpse and rose to his feet.

'Come on. We should keep moving. Hopefully, Gordon will have cleared the way for us some more up ahead. That young physicist is a one-man army as long as he has that HEV suit on him, and who knows what firearms he might have picked up.'

'Indeed, Eli. And let's hope the foreign entities he finds are no more dangerous than this fellow here.' He nudged the dead corpse with his shoe.