Just my guess about how the events of the particle accelerator explosion occurred on Earth 2 based on what we learned in Season 2. This will probably be a two-shot or at most a three-shot.

Scientific and Technological Advanced Research Laboratories

Earth 2

December 11th, 2013

The night the S.T.A.R. Labs particle accelerator went online.

The gold and chrome building towered over the crowd gathered before the recently constructed stage. There Harrison Wells, founder and CEO since 1991 would make his grand speech about the immense potential for progress the particle accelerator would provide. Caitlin Snow didn't know how smashing atoms together was supposed to help medicine or technology but she didn't much care. It didn't matter why a ruthless capitalist was flirting with wholesale planetary destruction, only that he was. The pamphlets about black holes, dark matter energies, and extradimensional aliens were burning a hole in the pocket of her winter coat. She fidgeted and for a moment rested her sign against her shoulder, the unsanded wood catching on the coat's fabric. The setting sun glinted off the top of a speeding monorail train and the bronze War of the Americas monument in the park across the street. Finally, it landed on the flecks of glass that Dr. Harrison Wells had sprinkled there as the concrete was poured. It really was a beautiful world, what a shame that all humanity ever wanted to do was destroy it.

They called her crazy. One of those anti-science lunatics, her ex-boyfriend's mother had muttered once. But it was science that taught her to fear the unknown. It was history that told her progress often came at the expense of innocent lives. Over the years she'd come to despise men like Wells, so consumed by the big picture to see the little people in it. They wasted their brilliant minds and millions of dollars smashing things together, blasting rockets into space, building future computer overlords instead of fighting disease or world hunger or social injustice or preserving the planet. They did what they thought was cool rather than what was right.

So Caitlin was there that night pressed against the police barricade, trying to shout over Wells who stood upon the stage with his daughter at his side speaking into a microphone specifically designed to drown out the protesters and make them inaudible. He spoke calmly, arrogantly about how that day would be remembered for generations to come. He was right of course but not for the reason he thought. At 9:36 pm, just four minutes into the particle accelerator's maiden run which had started two minutes behind schedule, an alarm sounded.

Martin Stein, Dr. Well's head of public relations, watched the computer screens flash red as champagne suds dripped down from the bottle in his hands.

"Dr. Wells? Please, tell me that doesn't mean what I think it means!"

Harrison Wells looked up and cursed. Shoving aside a research assistant he rushed to the controls. Reams of data scrolled past his eyes. He tapped in key code after key code, searching for, hoping for a mistake. There was none.

"Daddy?" Jessie asked, her voice full of a child-like fear she should have been ashamed of but couldn't bring herself to be. Harrison tried to ignore her, tried to concentrate. It just wasn't possible. Then, with a sickening thud, it hit him. They had been right all along. Those fools who'd questioned him were right. He was going to destroy the world.

"Dr. Wells? Harrison? What's happening? What exactly does that mean?" Martin Stein was in a state. His shock white hair stood on end and his face was turning from red to purple. Behind him the crowd that had been celebrating only moments before was frozen. They waited, fear caught in their throats at the look on their fearless leader's face. That was what shook him out of his stupor, their need for him to tell them it was going to be alright. God help him, how he ever ended up with all these people in his life? Who ever thought someone would look at little Harry Wells like that someday? Regardless of whether he deserved it or not, they needed him and he wasn't going to let them down.

"The particle accelerator is overheating but it's more than that. There's an energy that we never anticipated in the construction. Some type of radiation." A gasp went up from the crowd. They were scientists, biologists to engineers to physicists, they hadn't chosen their jobs for the excitement or the danger but because they believed in the project and because they believed in progress. Now it was hard to tell whether their horror was at the failure of their mission or the imminent possibility of the end of their lives. Jessie, her father's own Jesse Quick, didn't need to see the data to know the danger, the full implications of her father's words. She closed her eyes tightly attempting to convert fear into theorems.

Harrison turned back to the screens, commanding himself to think, think, think. Soon Henry Hewitt was behind him at the computer terminal so close he could feel his breath on the back of his neck.

"Dr. Wells? It's heating up down there, the thermal expansion alone could cause it to explode if we don't do something soon."

"Taking us and half the city with it," he responded as his gut clenched and sweat trickled down the side of his face.

Behind them glass shattered. The champagne flute tumbled from Martin Stein's cold, numb fingers then the sounds of running feet. Harrison turned to watch his friend run, knowing that he would not be back. Even if they lived to see the sunrise he would not be back. But there was a real chance they wouldn't, that another dawn would never stretch through his window and cast the window's shape on the floor. That if he didn't act now, none of them would survive. Martin would never even make it to the exit. The fear was enough to freeze his mind, trap him in his terror. Jesse, I can't let Jesse die. If everything failed this night that would be the last thought in his mind.

"Jesse, gather everyone. Take them to the time vault, they should be safe there. Henry stay with me."

The look on their faces was of complete incomprehension. The "time vault" as Harrison called it was not much more than a souped up storage closet, meant to preserve scientific innovations against the ravages of time but not a nuclear blast. Also it was clear Henry would much rather be there than at Dr. Wells' side.

"It's just a precaution. The shields, Jesse, remember the shields you recommended. They should contain the blast," his voice was maybe too sharp, his words coming too sharp. She shouldn't have believed the lie but he could tell she wanted to, was desperate to.

"But Dr. Wells…" Henry began but Harrison cut him off before he could finish the statement.

"Everyone, head to the time vault. Hurry!" He raised his voice, surprised by how sure he sounded. And like he'd spoken the magic words they came unglued. Plates and glasses were tossed to the ground and the group stampeded to the exit, certain of salvation unaware of how far off it remained. Jesse though didn't move, trepidation lurking in her dark eyes.

"Dad." He grabbed her shoulders, his little Jesse Quick. He would do anything for her. Absolutely anything.

"Jesse, please go now. Please." And she did with tears in her eyes.

Outside, Caitlin Snow watched the clock. 9:36 and nothing. The crowd still lingered. She wondered what they had all come out to see. Invisible atoms in a tube underground was not much of spectator sport. She had come for the cameras and for the man, Harrison Wells. Now he was inside celebrating and the news crews were packing up or sending out their final sign offs. The sun was set now and a storm brewed above them. She could taste snow on the wind. That was good, she had always loved snow. Ahead someone moved through the crowd with a predatory gait, eying figures around him like cattle lead to the slaughter. Caitlin tilted her head and watched him. He was for one a fine piece of work but that wasn't what had drawn her attention. It was the fire that glinted in his eyes. The energy with which he moved, barely restrained, almost as if he had trouble moving so slowly. She could tell even from where she stood that he was all heat. She almost licked her lips in appreciation.

Ronnie Raymond moved through the crowd with ease, gliding passed the suckers seemingly without a care in the world. He could hardly remember what this event was all about but it didn't matter. All that matter was the crowd and what sat in their pockets. A large man blocked his path and he had to turn sideways to squeeze by him. He leaned in, jostling the man just slightly more than necessary as his hand smooth as you like snatched the man's wallet from his back pocket. Ronnie felt its weight as he slipped it into his coat, fat. That was almost poetic or something. Already he had eight stored away and it looked like the crowd was dispersing. Time to call it a night. Well maybe after one last score. Ahead where the mass of people thinned there was a girl with a laptop bag, probably had her wallet in it too. Imagine what that can buy. Ronnie smiled, warm against the cold of the oncoming night. What a beautiful world.

Beneath the city, Francisco Ramon couldn't believe that this was the night he would die. Moreover, he couldn't believe that this was where he would die. Among the waste of the godforsaken city in the goddamned sewers. That was so beneath him. But the gun at his back didn't allow room for argument, didn't allow room for anything. It had a deadly certainty. He could tell that with the cold sweat running down his spine ran inevitability. It wasn't supposed to turn out like this but that seemed pretty much mute. There were things he shouldn't have done. People he shouldn't have messed with. But that was all in the past now and there was no future for Francisco Ramon. It was a shame, but what were you going to do? What were you going to do anyways? What were you thinking? He knew of course but in times like these, second guessing was a common problem. He had been thinking that he deserved better than this. That he was better than this godforsaken city, than this goddamned world. The universe's response, though, was apparently not.

9:38. Two minutes gone, maybe five remaining. That was so little time, so little time to make the most important decision of his life. Dr. Harrison Wells was used to having months, years even, to think things through. Now only minutes.

"Dr. Wells, the shields aren't installed. You said-"

"I know what I said, Henry. I know what I said."

The shields were Jesse's idea, something she suggested nearly a year ago. She was always bright, incredibly so but as she grew older he realized that already she was smarter than him. He could only imagine what she would grow to become. She was juggling nine college courses that semester but instead of working on that or maybe just after working on that she was reading his note. Especially the analysis of quantum mechanics that Hartley had started but never finished, from before the accident. She was concerned at the possible leakage of delta particles should the particle accelerator break a dimensional barrier when they attempted to break down an electron. She recommended installing shields made of neutron absorbers. There was a possibility but it seemed minute, less than one percent and there were construction costs to consider, investors to appease, a schedule to keep. Jesse didn't know yet about practicality. Compromises need to be made in the real world. Of course he'd told her he installed them, no need to stifle her dreams so soon. And now… now they were going to die for his hubris.

"Then what do we do?" Henry looked to him with fear in his normally jovial face. He didn't want to be here but he had to be. Harrison just hoped he could trust him with this the secret they were going to share.

"Open the release valve. Send the energy underground, into the foundation."

"But, the radiation… you can't- we can't. Who knows what it could do to the human body. You'd be introducing it into the water supply, infecting the city with it."

Something snapped in Harrison and suddenly he was holding Henry by the front of his lab coat, knuckles as white as the fabric. He shook the man, his face twisted to show a darker side he tried to hide.

"What would you rather do, Henry? Risk that or die. Because that's the choice. We send this underground or we set this city on fire. We blow out everything in a five mile radius and destroy STAR Labs. The building, the company and its reputation. You'll be dead and remembered as a fool and a villain. Your family will change their name to avoid being associated with the man who helped kill Central City. They'll say that about me too. Is that what you want? Tell me, Henry, if that what you want?"

The terror in Henry's eyes now was immediate, he was terrified of the man who stood in front of him. His hero who was turning out to be anything but.

"N-no, Dr. Wells. I don't want that," his voice was shaky, uncertain but he knew there was no denying this man and the fact was he was right. Already, Henry Hewitt knew the guilt of this choice would follow him to his grave but in truth there was no choice at all. One choice was horrific, the other unfathomable. No, no choice at all.

Harrison let out a breath, deflating. With some effort he detached his hands from Henry's coat and closed his eyes. There would be remorse but no regret. Jesse was more important than Central City, more important than the world.

The two of them were needed to turn the keys and open the release valve. To disengage the safety mechanism that prevented them from doing so while the accelerator was live. And then eyes meeting across the control panel of the Cortex, they twisted the keys. Harrison felt the click of the mechanism all the way up his arm. The ground beneath them lurched. The trigger was pulled at 9:42 on December 11th, 2013 and the world changed forever.