"Happy 2207 everyone!" Cheered a voice from the roof of the Garrison in ThreeTown. Northern Shield, the king of ThreeTown cavern and the first point of contact to the rest of Equestria, was standing at the top of the Garrison- a castle-like structure at one end of the massive cavern. As the cavern shuddered slighty, signalling the lift-off of another Equestrian spaceship, Northern waited for the ponies in front of him to quiet down before announcing, "Today, we begin our exodus! Equestria is finishing their final evacuation as we speak, and we're all next, on the underworld ship!" he said.
After getting a mixed response from the crowd, Northern leaned on the edge of the platform and said, in a more serious voice, "That's not all.... the heart of the earth, she stopped working one week ago. It won't be safe down here much longer, the planet's changes above us are affecting us down here just as much"
"Is that why all the other caverns have evacuated up to this one?" Yelled a mare from the crowd. Northern nodded and said, "Yes! The trains are going to start taking ponies to the ship we're using, so I would recommend you all start packing!"
In the crowd, a white mare sighed softly, looking around the crowd. "Hate space..." she grumbled softly to herself with a frown. She glanced at her watch, and gasped as she noticed the time, carefully edging back through the crowd to get back into her town, one of the three nestled in the huge cavern. As she walked home, hooves gently tapping against the centuries-old stone, she looked up at the old underworld houses on either side. Built of very, very old, dark red bricks, the identical houses dotted around the cavern had all stood for centuries, with the green crystal paths between them. And they had to leave it all behind? "What rotten luck...." mused the author as she rambled to her home, which was now temporarily home to two other ponies, too.
The mare's name was White Stream, and she was a well-known underworld author. Much like the writer of daring do above their heads, however White's adventures were mostly based in her head. She remembered the past year like it had been stamped into her mind. Just as the clock ticked over to 2205, harmony guardians such as the Tree of Harmony up above, and the Heart of the Earth down below seemed to just... die. Nopony knows why, either. Was it sabotage, from an enemy nation? Was it some monster from Tartarus? Discord went missing at the same time, was he related? Either way, the pressing issue was more important- the Tree of Harmony was done, and the Heart of the Earth had clattered and clunked to a dead stop, its gears and pistons frozen. With both safeguards ruined, the land soon began to fall apart. Cracks emerged in the ground, rivers overflowed their boundries, the weather began to buck control of the Pegasi, and earthquakes threatened to release Canterlot itself from its foundations! While some places outside of Equestria were suffering, it seemed that only the pony lands were experiencing such misfortune- the dragonlands and everywhere else were left alone, for the most part.
However, nobody would dare take in ponies once the magic broke- after a series of secret meetings between dragons, gryphons, zebras, and more, it was agreed- Ponies seemed to be cursed never to set hoof on their home planet ever again. For some nations, it was obviously born from their dislike of the ponies. And the others had either been convinced to their side, or threatened that way. Even if the rest of the world did want to take in ponykind, it would be a stress considering they were the biggest nation on the planet. It seemed that the "era of harmony" as it was named, which began in the 2010s, was now ending. It had come close to its end when Princess Twilight Sparkle's friends began to die, during the 2090s. Blessed with the long life of an Alicorn, the purple mare couldn't take the deaths of her friends, and she was driven to madness, and then insanity in her desperate pursuits to bring them back- Losing friends in such a permanent way as death was agony for the princess of Friendship, after all. She tuned to science, she turned to magic, she turned to dark magic, and after the death of Applejack in 2098, Fluttershy and Twilight both went missing.
Fluttershy and Twilight's bodies were never found, even though Applejack's body was discovered in the private areas of her castle. She was next to, and hooked up to, a running computer that had crashed sometime after her death. It's actually unknown why AJ was there, and what the computer was for. No data could be recovered from it, and when being examined it was proven that the elderly farmpony had been nearly dead at the time she was put in the bed. White had met the king's son, Polaris, and she'd asked him about Twilight one time. She remembered the look on his face when he explained to her his own meetings with the purple mare. He had told her, and she had refused to listen, that reviving a dead pony was impossible with magic. He'd broken up when he told White that Twilight knew he had tried it many many times, but she refused to believe it was impossible. She began to worry he would cry at his last line- "I don't know where she is, but if she managed to figure out that spell..."
"Ah!" White squeaked, as she managed to lose herself so deep in her thoughts that she walked straight into her front door. She fell down onto the step at the top, then giggled and rubbed her snout, tossing the door open once she got back up. As she trotted inside, she waved gently to her friends, saying, "I'm back~!"
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"Man, it sure would be nice if some unicorn fatass wasn't sitting around and helping instead" growled a voice. "Hey!" came another voice in return, followed by the laughs of a few stallions. In the center hub of the Underworld was a cavern named, accurately, "Centria". The massive train station in the center of the cavern had... seen a few things. Namely, a runaway train full of coal. Moving pretty fast. Not only did it jump the rails and destroy the massive turntable in the center of the station, but it also took down part of the building, and scalded some ponies with hot water, and lodged itself in such a way that removing it would bring down even more of the station.
"So, we done picking up everything?" Asked one of the ponies, as he leaned on his rather full bag. There was a small engine with a single coach behind it, and it was already halfway loaded with various things, mainly Everflame orbs, which the residents of the underworld had discovered were impossible to make without the heart's magic- therefore they vowed to take as many as possible with them to the stars. "Yeaaaah, just about... we all here?" Came another stallion's voice.
"Think so, let's buzz off, we've just been sittin here for a while" grumbled the first one. The small team of workers piled into the carrige, and with a hiss of the engine, they took off to ThreeTown for the last time, leaving Centria in dark silence.
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Each cavern except ThreeTown had fallen apart without the heart's magic, and the aforementioned one was only going to stay together for so long being held up by layers and layers of magic wards placed in desperate attempts to keep it together. Crystal Cavern had suffered the most horrible death, and was the first to collapse. The massive crystal city in the center of the town had collapsed into its ocean, sending earthquakes through the Underworld and Equestria itself. Crystal Cavern was also the only one to collapse entirely- bringing down part of equestria- taking Twilight's Castle down into the earth with it. The plantling hive, thankfully, managed to successfully evacuate after the crystal city had collapsed, since it had flooded part of their hive.
Next, came FallingRock. This cavern didn't suffer too hard, simply put, the small town within had been demolished by rockfall from Crystal Cavern's collapse. Perhaps the name had been a little too foreshadowey. After that, came Greenwall cavern. First, the mindvine died. Most of the ponies in the Underworld had never even considered the possibility of the sentient plant dying, and yet it simply shriveled up one day at an alarming rate, and then it began to rot. After that, all the houses held up by the sentient plant collapsed, and after the rest of the crops died, filling the cavern with deadly mold, it was then abandoned too. Soaring Cavern? Catastrophic failure, thanks to earthquakes, most of the massive Stalagtites lost their grip, smashing down into the cavern's ocean. Skytown was ruined with rubble, its hangar having fallen into the drink, and seatown was flooded with the Underworld's first and only Tsunami.
DarkRock suffered the worst fate- Infinity City's floodwaters broke through into the cavern, and the magma that once ran the heart of the earth bubbled up into the cavern, ruining it and burying the town under a nasty mix of molten lava and boiled water that cooled into stone which encased most of the town. Even Gemstone Town had suffered a cave collapse, the town buried under tons of collapsed rock. However, that disaster had a silver lining- the ghosts who were confined to its borders for centuries were set free when the town was buried, and they moved on to ThreeTown with the others. They would be the first ghosts in space!
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A tall, white mare cleared her throat firmly as she frowned down at the nervous stallion in front of herself. "Repeat that again, Engineer?" She quietly said.
"H-haha, I-I uh, well you see, the... the...." he said, gulping quietly as she continued to stare down at him. "O-Okay, so, t-the SS Twilight! The uh, the old superfreighter! The uh, we have a problem...."