Prompt is inspired by this gif from the Reddit post with the same name.

Mark was an assistant at the PPK Lab for quite some time now, working there while undergoing his studies as a medical engineer. He'd usually work on compartments in the lab, and was rarely a part of the "bigger action" as Mark himself said it - he was never able to test his engineering, he wasn't allowed since, in the PPK boss' words: "You're just an assistant."

So it was quite weird for Mark to receive a call about testing a new project the lab had been working on, in secrecy. They called it the "marvel of science", or that's at least what was written in the e-mail Mark got the night before.
Arriving in the lab, he was given the directions of this new project, this new experiment. Eager, Mark rushed to the room, and as he opened the door, he expected a high-tech interior, with antennas that could communicate to aliens, or shuttles that allowed humans to travel to Mars.
Instead, Mark's eyes got greeted by a garden, something like a common house owner would have in his backyard. And in the middle, a peculiar object made out of plastic fabric greeted Mark, coldly resting in this weird room of the laboratory.

"...a fucking kiddie pool?" Mark said audibly.
"Out of all things I get to test for the first time, it's a pool made for newborns?"

Laughter could be heard, just barely, as it was coming from the control room.

"Not just any kiddie pool!" a voice exclaimed. Mark could see a person coming out of the control room, and it was Justin, one of the scientists of the PPK Lab.
"But a magical kiddie pool!", he added.
"This has got to be some joke, right? I woke up early because of this." Mark responded.
"Not a joke at all! We found this kiddie pool very near our base, and since you're one of our best assistants, I found a way to make you a part of the project. You need to have some fun, too." Mark was flattered, but still bewildered by the entire situation. A magical kiddie pool? He had seen many miracles in this workplace, from plants and small animals being revived from death, rats being able to live without vital organs, and shifting and bending materials to what seemed humanly impossible. But this? Mark found it hard to believe that someone left a pool outside of the laboratory.

He embraced the absurdity and continued on.

"So, what do you want me to do, exactly?"
"Well, this morning, while bringing this pool to the lab, we played around with it a little bit, and I dropped one of my pencils into it. And, it disappeared. Reaching my hand to pick it up, it somehow extended way farther than the pool's depth, and I still couldn't feel the surface. So, I want someone to fully submerge themselves in the pool."
"You want me to be a test subject???"
"Exactly."

Mark couldn't believe it. At first he didn't want to do it, but he was way too intrigued by the concept. A kiddie pool that was bigger on the outside? He chuckled to himself. Now that was a sight to behold.

Mark went to the control room to prepare for the expedition. Along the way, Justin explained what he was supposed to do.

"We have prepared the fitting attire for this experiment. You're gonna get changed, and we'll equip you with an ear piece, along with a rope that's gonna be attached to your waist, so we can pull you out if there's any need to. You'll just tell us what you see."

Mark got into the changing room that was present in the control room. He put on a pair of swimming shorts, flippers, a mask and...

"Are these really necessary?" asked Mark as he grabbed a pair of arm floaties.
"You never know what's lying beneath. And plus, it's a kiddie pool, who knows, it might not even let you in without one." answered Justin, sarcastically.

Once he was ready, the other scientists gave Mark the ear piece and hung a rope on him, as he was told. He got out of the control room, and went to the kiddie pool.

"Test, test, test." a staticky voice could be heard from the ear piece.
"Hear you loud and clear." responded Mark.
"Just to repeat what I told you already. Once you step into the kiddie pool, we want to hear what you see on the other end. As soon as we're pleased with the information we get, we'll pull you out. Or in case something dangerous happens, too."
"So... how do you want me to enter this thing?"
"I don't know, stand in it, jump in it, pretend that you're jumping off a diving board. Whatever floats your boat."

Mark was eyeing the kiddie pool. He still couldn't believe the whole ordeal. Wearing a pair of swimming shorts, with flippers, and fucking floaties, he thought. But, he composed himself, and first reached his hand into the pool. He was surprised to see that Justin was, indeed right, as his whole hand was shoved in it, and he couldn't feel the end.
So, Mark embraced himself, as he jumped, and lunged his whole body into the pool.

Water embraced Mark's vision. He was holding his breath tight. He couldn't believe it. One second he was in the sight of the laboratory, and now it appeared that he was under the sea. Or an ocean. Some body of water, he didn't know exactly what it was. The water wasn't salty, nor did it have the taste of regular water, it was something in between. He tried keeping his composure, and luckily he wasn't that far off from the surface. As he swam up, he looked around himself, and was presented with the vast nothingness of the water. No fish, no coral, and no surface. He wanted to see where the rope was, and sure enough it was still on him, he just couldn't find the end.

Mark got to the surface, and was gasping for air as he tried to float on top.

"Mark? Is everything alright? Can you hear us?" the voice from the ear piece arose.
"Yeah... yeah... I'm fine." Mark answered, trying to catch his breath. "Hearing you loud and clear. I got teleported underwater and had to swim to the surface, it was a long way up."
"So, what do you see? In what body of water are you in?"

Mark looked around him. And no matter how much he squinted, nor how far he looked, his vision was presented with just water. Water and nothing else for miles and miles, no surface whatsoever.

"Well, imagine you're stranded somewhere in the middle of the ocean, that's what it feels like right now. I see just water. And, frankly, I couldn't tell you if I'm in the ocean. The water doesn't taste salty. Nor does it taste like water. Imagine you're drinking still, warm water, with a touch of salt tingling your tongue, trying to force the taste in your mouth. It's like that, but the water is cold. That's how the water tastes." Mark tried to explain the taste as best as he could. It really was strange to him, this water he was in. It seemed like the kiddie pool holstered an entire different universe. He felt uneasy being here.
His thoughts would be quickly broken by the ear piece.

"Interesting, very interesting. What about the rest? What do you see in the sky?"

Mark looked up. The sky was clear, with a shade of bright blue, with no cloud in sight. Along with it came the sun, bright as ever, touching the horizon as its flares danced around the body.
Mark reported what he saw, and was greeted by a very surprised response.

"A sun? You see the sun?" scribbling could be heard on the ear piece, "Like ours? Are you sure you didn't get transported somewhere on Earth?"
"Justin, how would I know? Being in water... doesn't really give me anything."
"Fair, but that's just it? No waves? No creatures? Weird."

And it was weird. There was a cool breeze hitting Mark's shoulders but that was about it.

Mark stayed for a couple of more minutes in the water, just to see if anything would happen.

"There's an oncoming wave, headed towards me." reported Mark.

He thought it would be nothing special, that's how a body of water functions, but it was nice to see at least something happening. Mark was quickly getting bored and he stood in the water with no activity whatsoever.

What Mark didn't predict, though, is when the wave hit him, the force of it was so strong it pulled him deep underwater. Gargling noises could be heard, as Mark was surprised by the sheer strength of the wave, and how deep he went under.

"Mark? Mark? Are you alright? Do we need to pull the rope?" Justin asked, concerned. A distant, but audible "nuh uh" came from Mark, trying to make sense of what just happened.

Concerningly, he just kept descending deeper, to the point where the surface was not visible anymore and darkness surrounded his vision. The coldness of the water was getting more and more intense, and the pressure slowly intensing.
Mark kept screaming "Mhm! Mhm!" in a way for him to get out of here and back into safety.
Back at base, everyone was monitoring the sounds at this point. They could detect the sheer fear in Mark, and the tipping point was when they heard muffled screaming, along with a low-pitched screech that came out of the ear piece. Justin ran toward the rope, and pressed the emergency button that pulled it as fast as it possibly could.

Thankfully, as they all ran towards the kiddie pool, they all saw Mark flop out safely, holding tight onto the fake grass, as he was coughing, almost puking, the water out of his body.
Justin ordered for the emergency services to aid Mark, as he ran towards him. He was fine, he could breathe, but that expression. The expression of a man who saw something he shouldn't. The gaze of a man who met death, in front of its doorstep.

"Eyes, big red eyes. It had giant red eyes. In the water. In the deepness of the water. And that mouth, that giant mouth with razor sharp teeth. And the noise it made..." was all Mark could say before he passed out.

Emergency services arrived and did their best to save Mark, as Justin's attention caught onto the pool.

"What the...?" He questioned, as the water inside of the kiddie pool was turning into a shade of black. His eyes filled with terror as a whirlpool started forming, the water somehow getting darker, and darker.

He saw something, he saw the same two red eyes that met Mark. Impossibly huge, impossibly huge to fit in that pool. He swore it could move.
He embraced for the worst, but the eyes disappeared and the water went back to its titular transparent blue color, the same color as they found it this morning.

The threat was gone, Justin thought, but he knew, he had a hunch, that they weren't safe.

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