Dipper Pines sat quietly at the top of a hill, watching his twin sister Mabel roll down the soft green grass.

"Mabel," he said finally as she walked back up the hill, grass scattered across her sweater, "Why do you keep doing that?" Mabel rubbed her chin, pretending to be thinking.

"Oh I don't know, maybe I just wanted to do-" she reached out quickly and shoved Dipper, sending him tumbling down the hill. "THIS!" She grinned and rolled after her brother.

Dipper bounced down the hill, crashing in a small field of wildflowers at the bottom. Mabel trailed behind him, bouncing and landing on Dipper's back.

Dipper groaned. "Mabel, what was that for?" Mabel shrugged with a smile, flower petals littering amongst the penguin design on her sweater.

"I dunno, maybe I was testing gravity." Dipper rolled his eyes and pushed his sister off his back. Mabel stood and brushed off her sweater, sending a few petals flying. She then proceeded to twirl carelessly around the field.

It must be nice to have little brain capacity, Dipper thought sarcastically, taking a seat in the field. A little bare patch of dirt sat in front of him. Huh. I wonder why there's no flowers here.

He watched, bored, as a couple ants crawled by his hand. He ran a finger through the dirt, stirring up a little hill that blocked their path.

He raised his eyebrows and drew a few more grooves in the dirt, making a little maze for the ants. He continued to draw in the dirt, spreading the dirt out in front of him.

Suddenly, a sharp pain coursed through his finger. "Ah!" he exclaimed, holding his finger to his face. A little sliver of wood poked out of the tip of his finger. "A splinter?" He winced and pulled it out slowly. Dipper looked at the ground curiously.

"How did I get a splinter? This is all dirt and flowers." His eyes followed his little dirt path, where the ants were crawling over his drawn paths. He looked at the spot where he'd stopped.

A little bit of wood stuck out where Dipper had caught the snag in it. His eyes widened as he brushed some dirt off the top of the bare patch in front of him.

Boards of wood appeared where the dirt had once been. Dipper pushed some more dirt away, revealing a rusted metal handle.

"Whoa," he said, lifting the handle. It was heavier than he'd expected. He stood, still clutching the handle, and pulled the door open. Cobwebs fluttered out into the breeze, and a cloud of dust flew up to meet Dipper.

He coughed and waved the dust away from his face. Opening his eyes, he was surprised to see a worn wooden staircase leading underground. Water dripped from somewhere inside the hollowed ground. Dipper's eyes widened.

"Mabel!" he yelled, scanning the field for his sister. She was nowhere to be found. Dipper gripped his cap nervously. "Mabel?! Mabel?!"

"BOO!" Dipper screamed and turned to see Mabel crouched down in the flowers behind him, laughing her head off. "Oh man, you sure are easy to scare!" Dipper rolled his eyes and pulled Mabel to her feet.

"Whatever," he said, pulling her to the trap door, "Look at this." Mabel gasped and smiled.

"Cool!" She paused. "What is it?" Dipper pressed his palm to his forehead.

"It's a trap door, Mabel." Both kids leaned closer to the door. "What do you think it leads to?" Mabel started to walk down the old stairs, each creaking in protest with her every step.

"Only one way to find out," she said, marching lower into the ground. Dipper's eyes widened and he grabbed Mabel's arm, pulling her back up to the field.

"What are you doing? It could be dangerous!" He looked down at the staircase. "We don't know what could be down there!" Mabel rolled her eyes and pushed her brother.

"Since when has that been a problem before?" Dipper looked at her blankly. Since when have you made a point? Dipper sighed and gave her a determined smile.

"Okay, you're right, but we should at least tell Grunkle Stan where we're going." Mabel rolled her eyes.

"Okay, fine," she said hesitantly. The two raced through the woods, stopping at the Mystery Shack. There, they sped into the gift shop, where they scribbled a quick note to their great-uncle.

"Okay, you ready?" Dipper asked, setting the note on the cash register, where they knew Stan would find it. Mabel, who was lying on the couch, shook her head. "What? Why not?"

"I'm pooped. I can't run through the forest and all the way back there! It's like, too far!" Mable rolled over, tumbling off the couch and onto the floor. "Suddenly I feel much better."

Dipper looked at his sister curiously and glanced out the window. Outside, he could see Wendy parking the Mystery Shack's golf cart. He smiled deviously and glanced back at Mabel.

"Hey Mabel, I think I might need to leave another note." She looked at him. He tilted his head towards the golf cart.

"Awesome!" she said with a smile, standing and running excitedly towards the door. I knew she'd approve.

Dipper and Mabel ran outside to meet Wendy as she headed inside. She smiled at them as they rushed up to her.

"Whoa guys, where's the fire?" Dipper froze, getting nervous around the teenager.

"It's...um...I mean...um," Mabel pushed in front of him.

"Can we use the golf cart?" Wendy smiled and tossed Mabel the keys.

"Try not to destroy anything!" She called as she walked inside.

"No promises!" Mabel called back. Dipper stood behind Mabel, still in a slight daze. Mabel picked her brother up by the waist and sat him in the driver's seat of the cart. She then climbed into the passenger's seat.

Dipper stared blankly ahead of them, still in his daze. Mabel rolled her eyes and slapped him hard across the face. He tensed and shook his head.

"Whoa, what happened?" He rubbed his cheek. "Oh, wait. Wendy?" Mabel nodded. Dipper groaned as he buckled his seat belt and put the cart into gear.

Poor, lovesick, little Dipper. Some get distracted so easil- hey cool! A gopher! Dipper drove them back to the little field, where he parked the cart by the small trap door.

Standing above the door, he took a deep breath. Mabel walked up to meet him. She looked at him.

"Ready?" She replied with an energetic smile. Dipper gave her a small smile back. "Let's go see where this takes us."

They say curiosity killed the cat. Good thing I'm not a cat.

Dipper went first, every step he took audible enough for Mabel to hear. She stood above the door, watching as her brother descended into the earth. "Do you see anything?"

"Not yet!' Dipper called up to her. "Shoot, I wish I'd brought a flashlight."

"Don't you keep one in your hat?" There was a short silence.

"Oh yeah! Thanks Mabel!" Mabel saw a yellow beam of light flash around underground.

"Do you see something now?"

Dipper looked around. The stairs had led him down into a dark, dank cave. Water dripped from stalactites, and sharp stalagmites stuck up through the gray stone floors. Dipper suddenly spotted something.

A yellowing piece of paper sat on the ground in front of him. He carefully picked it up.

Dear Reader,

Welcome to the Gravity Falls Caverns. Beware; some say that you will meet more than roaches and bats in these twisting tunnels. Watch for-

The letter stopped. As Dipper examined the paper, he saw that the end of the paper was ripped. The jagged ends elongated until Dipper could trail them up the paper.

These look like...claw marks! Dipper's eyes widened, but he didn't run back to Mabel.

"Dipper?!" He snapped back to attention.

"Uh, yeah Mabel! I see something!"

"Where? I wanna see it!"

"Come down here then!" Dipper heard loud, fast footsteps as his sister raced down the steps. Once, he heard a slipping sound and Mabel's shriek. "Careful, the steps are wet."

"Oh now you tell me?!" A few moments later, Mabel stood at his side, reading the letter.

"So, should we keep going?" she asked him. Dipper looked at the rest of the cave. It led into a dark tunnel. He then looked at Mabel, who studied the tunnel.

Dipper finally shrugged and said, "Why not? There could be something interesting down there." The twins walked towards the mouth of the tunnel.

"Dipper, I'm not sure about this."

"Come on Mabel, think of what we might find down there?"

"Hmm, let's see, our demises?" Dipper rolled his eyes.

"Aw, come on! What happened to the girl that loved to monster hunt?"

"She's still here! She's just...scared." Dipper threw his arm around his sister.

"Mabel, think of it! What if we exposed an actual monster to the people of Gravity Falls?! Grunkle Stan would get more business, which means that we might actually get paid for working there! I'd get international fame, we'd get fortune, boys would be all over you," Mabel giggled at this part. "And, you could finally get that Hamster Ball you wanted!"

Mabel smiled. "What are we waiting for?! Let's go get that monster!"

"For fame!"

"For boys!"

"For money!"

"For hamsters!" They laughed and raced into the tunnel, Dipper lighting the way with his flashlight. Glimmering stones appeared as Dipper's flashlight brought out their wet shine.

Slowing down to a walk, Dipper began to notice some strange things on the walls of the cave.

"Hey Mabel, check this out." He pointed to a strange groove in the rock.

"Are those...claw marks?" Dipper didn't have time to answer. An angry roar echoed through the cave. It sounded like it came from behind them.

"Dipper?"

"Run."

"What?"

"Run!" Dipper dashed down further into the tunnel, Mabel following him. They heard heavy breaths and scraping noises behind them. They seemed to be getting closer.

The twins came to two openings in the tunnel. "Which one do we go through?"

Dipper glanced behind him. An immense shadow reflected off the cave walls. It was getting close to their spot.

Dipper quickly pointed to the tunnel on the left and grabbed Mabel's arm. Dragging her through the tunnel, he quickly checked for their pursuer. To his horror, a huge shadow followed them, running on four legs.

The creature gave an unearthly howl and suddenly charged forward, coming so close that Dipper could feel it's hot, tired pants on the back of his neck.

"What is that thing?!" Mabel called, running a little faster.

"I don't know, but it sure doesn't like tourists!" The twins sprinted through the caves, the tunnel making swift twists and turns as they coursed through the ground.

There's gotta be a way to slow him down! Dipper glanced down at the flashlight in his hand.

"Mabel! I'm gonna try something, but you have to keep running, okay?!" Mabel looked at him like he'd suggested that she stop wearing her sweaters.

"What?! Dipper, I'm not leaving you to deal with that beast!" The creature gave a roar. "No offense." It barked.

"Mabel, it's the only thing I can do to keep it from you!" Mabel looked ahead of her, then back at her brother.

"If you're gonna keep it from me," she said finally, "Then I'm gonna keep it from you!" Dipper smiled gratefully at his sister.

"Okay, so, at the count of three, I shine the light in the monster's face." Dipper suddenly frowned. "I wish we had a camera. How are we gonna get any proof of this creature?"

Mabel thought for a moment. The creature let out another bloodcurdling howl. She suddenly brightened and lifted Dipper's hat. A small camera sat between his locks of brown hair.

"How'd you know I had that?"

"I pack your cap every morning to make sure you're prepared." Dipper blinked.

"Really?"

"Yep!" He smiled at his sister.

"Cool, thanks!" He looked forward. "Now let's do this." He tossed Mabel the camera. "Ready?"

She hesitated slightly, but nodded. "One."

"Two."

"THREE!" Dipper whirled around, his flashlight shining directly onto the beast. He almost dropped it as he came face-to-face with an enormous black dog. It had fangs dripping with black ooze that looked suspiciously like venom.

It's eyes glowed red, and it stood a good four feet above Dipper. Long black claws stretched out from each of it's paws, extended and sharp enough to rip him apart with one swipe.

The beast recoiled back at the sight of Dipper's flashlight. It must hate light! It's lived in the caves so long, it probably hasn't seen light in ages! Dipper held it closer to the monster's face.

"Mabel! Get a picture!" Mabel raced up to the demonic dog and snapped a picture. She frowned at the camera."

"Ugh, it's too dark. I have to turn the flash on."

"TURN THE FLASH ON! TURN THE FLASH ON!" Dipper glanced at his flashlight, which began to flicker. The beast growled, it's venomous fangs bared menacingly.

"Hold on, almost got it."

"MABEL JUST GET THE PICTURE!"

"Aanndd..." Dipper's flashlight went out. Then Mabel's camera flashed.

In the dark, Dipper saw the demon dog recoil back, stunned by the bright flash.

"Hurry Mabel!" He pushed his sister in front of him. "Let's go! It's flash-blinded!" He hurried in front of Mabel, dragging her through the caves once again. They rushed through the tunnels, finally coming to the mouth of the cave where they'd started.

Mabel scrambled up the ladder and into the field, where dusk was just beginning above ground.

Dipper stopped at the spot where he'd found the yellowing note. It was lying crumpled on the ground.

"Dipper, come on!" Dipper ignored her and broke a piece off a stalagmite. Scratching on the cave walls, he wrote, BEWARE OF CAVE DEMON. He could hear the dog charging after them.

"DIPPER!" He looked at his sister. "LOOK OUT!" He turned to see the demon dog standing above him, venom dripping to the cave floor and burning holes in the stone.

The dog raised a paw and brought it down hard near Dipper, snagging his jacket collar and pinning him to the ground. It raised another paw and dug it's claws into Dipper's arm.

Bright red blood leaked from under the beast's paws. Mabel watched in horror as it leaned closer to her brother, venom dripping from it's fangs and burning holes in his clothes. Wait a second, why am I just watching?!

Dipper watched as Mabel ran away. Coward. I'll miss that. He tried not to look at the menacing beast towering above him. He felt a hot liquid burn through his cap and singe a part of his hair.

Making one last attempt to struggle, he squirmed under the monster, which pinned him down harder. Pain shot through his arm as more blood leaked from under it's paw, dripping onto the cave floor.

The beast seized Dipper by his collar and lifted him into the air, slamming him down on the solid stone floors of the cave. He suddenly felt lightheaded.

It raised it's head again, throwing him onto the floor again. He could feel his head and eye throbbing, and he knew for sure that he would have a black eye...if he survived.

The beast raised it's head on final time as Dipper prepared himself for his death sentence. Goodbye Gravity Falls... The beast lowered it's head.

Then Mabel crashed through the trap door. She was driving the golf cart with Wendy, Soos, and Grunkle Stan sitting in the back seat. The headlights shone through the dark cave, blinding the monster and making him drop Dipper.

"DIPPER!" she screamed.

"I got him!" Soos yelled, leaning out of the cart. Dipper's limp body fell into his arms.

"It's gonna be alright dude," he said, trying to calm the shaking boy.

"It's hurts," was all he managed to say. Meanwhile, Mabel rolled out of the cart.

"Wendy! Take the wheel!" she ordered. Mabel then ripped a wooden board off the staircase and held it at the monster. "HEY UGLY!"

The beast growled at her. She didn't flinch. "No one. Hurts. My. BROTHER!" She swung at the dog's legs, making it collapse with a thud. She beat it a few times across the face with her makeshift weapon, earning it a few splinters in it's nose.

The beast writhed and swatted at Mabel, sending her flying into the golf cart. Wendy smiled at her and pulled out a baseball bat. She smiled gratefully and went back to face the monster.

Soos and Grunkle Stan sat leaning over Dipper, the latter holding a wet cloth to his eye while the former wrapped his arm in a bandage.

"And I thought the First Aid Kit was a waste of money," Stan said, flipping the towel on Dipper's eye.

Dipper sat up shakily. A cut on his head started to bleed, as did many others on his arms. A skinned knee burned as he shifted in his seat.

"Dude, be careful," Soos warned him, patching Dipper up with more bandages. Dipper ignored him.

"That's my sister," he said, dazedly watching Mabel dodge the beast's attacks and effortlessly beat the tar out of it.

The beast had open cuts and was clearly in pain from Mabel's attacks. It had slowed down greatly, and Mabel used that to her advantage, jumping up and giving it a hard whack to the face.

Wendy, Soos, and Grunkle Stan winced. "Ooh, that's gonna leave a shiner," Stan determined. Dipper was wincing already from the stabbing pain in his head.

Slightly blinded, the creature stumbled backward into the caves, whimpering with it's tail between it's legs.

"Yeah, you better run!" She had survived that battle with nothing more than a small bruise from slamming into the golf cart.

She ran back to the cart. The others cheered for her, but she ignored them and climbed in next to her brother.

"Hey Dippy," she said softly. He smiled weakly at her. Wendy drove effortlessly up the stairs, to the dismay of Soos and Stan. As she turned through the field and onto the main dirt road leading through the forest and back to the Mystery Shack, Dipper looked up at Mabel.

"Mabel?"

"Yeah?"

"Never push me down a hill again."