The door to the sea train slid open, and the familiar smell of salt hit his face immediately. As people began to pour out, he followed along initially, flowing with the rush of the crowd, not wanting to be trampled or wind up accidentally going to the next stop because he wasn't quick enough. He heard a soft ding behind him, then a loud hiss as the sea train sealed itself airtight. Now with the crowd mostly dispersed, he began his bi-weekly ritual of turning around to watch it leave. It floated just inches from the surface of the water normally, but rose a few more inches before accelerating quickly forward. It was out of sight within seconds, leaving him almost completely alone on the platform.
Keanos, the fifth planet from the sun of the star known as TRAPPIST-1, was a majority ocean world, somewhere in the range of 99.5% to 99.9%. On a world nearly 1.6x the size of Earth, that meant that you could jump into a boat and cruise, and never so much as see a single thing resembling land, or a dry surface that wasn't man made. What little land there was to be found were rocky, jagged stalagmites sticking out from the ocean, usually far too small and thin to put anything with any real weight on it despite being made of igneous rock, and often disappearing and reappearing with the tide. The ones that were large enough to be used were occupied by ocean research outposts and platforms, manned by skeleton crews.
For the rest of the world, floating cities were erected where people lived, though not a ton for a world as large as Keanos. There were eight of these floating cities, four for each hemisphere, all more or less equidistant from one another, all roughly the size of Manhattan. That is to say, about 23 square miles, give or take a mile, and modern technological marvels. They ran on a mix of solar and geothermal energy, each city tapping into the planet's core for the majority of its energy needs. They'd thought of just about everything, all the way down to making the city capable of launching high into the air to avoid Keanos's incredibly powerful mega-storms that would sweep through somewhat regularly. Then, when the threat had passed, it could land gently as if it had never moved.
Rylan was one of about a million men and women currently living in one of these cities, better known to the people of Keanos as Sea Castle, located in the northern hemisphere. Sea Castle had one very important job, and that was to build both manned and unmanned starships and submersibles for space and ocean use. Prior to the discovery of anti-grav technology, humans were content to build their starships in space. It cost far less to ship things up via space elevator, then put them together with the help of drones, as opposed to the old way of building everything on the ground. Anti-grav technology basically allowed humans to laugh in the face of physics, and use a resource-rich planet like Keanos to its fullest. Launching extraordinarily heavy starships full of supplies suddenly became extraordinarily cheap, so building them on site where the resources were mined, refined, and then turned into things for manufacture, was the smartest thing to do.
The other cities played a role as well, usually mining operations and manufacturing. All spectacularly boring affairs, making Rylan glad that he wasn't stuck working in those industries. No, he was a marine biologist, sent to Keanos several years prior to study the ocean life in its many and varied forms. He worked with one of the aforementioned skeleton crews on a floating research station, a fairly small affair that used all of its space incredibly efficiently. Just enough room for experiments, not much room for anything else besides the absolute necessities.
Unfortunately, this was an on-week for Rylan. Marine biologists on Keanos worked on a two-in, two-out system, whereby each team of three swapped in and out every two weeks, taking some time to leave a brief for the incoming team on any changes and potential challenges for the experiments they were given, or research specimens, depending, and then were left to their own devices. He didn't really mind that he had to work in such long stints, but it was rough to make friends, or potential lovers, when you worked half of the year in two week stints.
Rylan walked across the now empty platform to a tiny little booth. Inside the booth was a slightly rusted robot assistant installed into said booth, who didn't really have much of anything from the waist down. He wasn't a fan of these particular robots as they were pretty much faceless, with minimal openings and joints, so that the salty air couldn't get into them and wreak havoc on their sensitive insides. Sadly, he had to endure talking to this thing at least twice every two weeks. Once on arrival, once on departure.
"Good morning, Mr. Larsen," came the synthesized voice of the robot. "Is it that time again? I feel like we'd seen each other only yesterday!"
Rylan sighed lightly. It was the same joke the robot made every time they interacted. Part of him wondered if it knew anything else, or if this was something other marine biologists had to endure. "I'm here to take the Blue Marine to Brava Platform. Can you summon it for me?"
"O-o-of course, Mr. Larsen. All fees are waived for employees of the Cousteau Marine Institute, as I'm sure you know."
"Yes, I'm aware," Rylan groaned lightly, pulling out a small tablet from his pocket and keying in his security code, a twenty digit sequence of numbers that only he knew, and he waited patiently. Submersible storage was often held underwater, since building upward wasn't realistic. Luckily, each submersible was sealed in its own little thick plastic bubble so as not to gain any hitchhikers. He watched as the plastic bubble rose slowly to the surface and opened. His submersible, a light blue, two person craft, was held in place by robotic arms in the bubble, allowing him to enter when the top half unsealed and slid away, allowing him access.
The Blue Marine was capable of being entered from just about any angle. Front, back, bottom, top, or sides. This was both a safety feature and a convenience, allowing the possibility of escape in emergency situations where the submersible would otherwise be trapped, while being convenient to enter based on whatever task it was being used for. The entire front half was translucent, made from a thick see through super-plastic designed to withstand crushing ocean depths. It couldn't quite go down however deep it wanted to, but generally speaking, it could easily handle depths greater than the Mariana Trench on Earth, around 11,000 meters, without so much as a creak or groan. Not that you'd want to normally on a dangerous world like Keanos.
Rylan walked forward as a small platform extended out to his submersible's plastic bubble and he carefully hopped atop the submersible waiting inside. He unsealed the hatch on top, slipped in, and closed the hatch. He double checked the seal to make sure it held, and then began a pre-dive check, a number of diagnostic scans and engine tests to ensure he wouldn't be dead in the water. When he was finished, confident the vehicle would perform as necessary, and sealed tightly within, he sent a signal to the robot up above. The bottom half of the plastic bubble slid away, and the robotic arms holding the Blue Marine dropped him unceremoniously a few feet to the water's surface below.
The moment the submersible hit the water he checked to ensure there weren't any leaks. He had to check the front visibly, but there were censors in the other areas that would've warned him of any leaks. He still took a second to listen, then turned his attention forward. A number of fish, both carnivorous and otherwise, had already begun to swarm his craft. The ones that weren't were attempting to find and pluck parasites off the ship's hull, while the predators were eager to take advantage. If not for the protection of his craft, Rylan would've been picked clean apart by the predators in seconds. One of many reasons why swimming on Keanos was strictly prohibited and tightly controlled.
Confident, Rylan gunned the accelerator and aimed the craft downward, using its ability to change color to have it automatically detect the light and get darker to blend in as he dove deeper. The goal was to aim between 800 and 1000 meters, toward the bottom quarter of the Mesopelagic Zone, a depth of ocean known more commonly as 'The Twilight Zone' due to its rapidly decreasing light the further down you went. At these depths, he was able to avoid Keanos's apex predator, referred to by anyone who had to travel the oceans as the Charybdis, like the mythological creature of Greek origin.
The Charybdis was to be avoided at all costs, an enormous eel-like creature that traveled the many similarly large lava tubes that dotted the ocean floor. It used these tubes to travel within the planet to various feeding grounds. Normally, the thing was only about a fifty to a hundred feet wide depending on the age of the specimen, but it had one particular feature that made it a very successful predator and also earned it its nickname: when it opened its huge toothy mouth, it spread it out many times wider than its own body like a net. It would do this, lunge forward to take hold of whatever it thought it could eat - entire schools of fish, large whale-like creatures, even human-manned submersibles - and then dragged it all down to the planet's lava tubes to digest. In the early days of human exploration of Keanos, that was the most frequent way exploratory vessels disappeared.
However, it was later found, thanks to the brave marine biologists assigned to study the Charybdis, that it had more traditional eyes and hunted by movement and detecting light, like fish scales reflecting the sun. Therefore, subsequent vessels created were fitted with technology that changed their outward color to match the depth, filtered any outgoing light, and anyone traveling were mandated to travel below 800 meters for nearly the entire trip to minimize risk. You had to also be vigilant, as bumping into a Charybdis would also get you quickly swallowed and dragged into the depths, so the auto-pilot was disengaged almost every time.
To say the least, the Charybdis was far from the only oceanic predator on Keanos, just the most notorious and successful one.
There was a wide variety of fish down in the Twilight Zone, all bioluminescent, all of them using a sort of echolocation to find food or avoid becoming food. From time to time, he could hear a soft click and see a streak of light jet away. He dove a bit further down until the light was completely gone, and it was like driving his submersible through a sea of stars of all different kinds of colors, shapes, and sizes. At one point, he felt a firm bump, and then a set of long tentacles appeared from below, wrapping slowly around the vehicle. It was one of Keanos's predators, but this one was too small to be of any real threat. He kept a finger on the button meant to deliver an electric shock to anything touching the outside, but when it released him, he relaxed.
As he neared the little research platform he was to call home for the next two weeks, he began to ascend. It didn't take long for the underwater docking bays to come into view. They weren't at all like the ones back home at Sea Castle that he often used, those plastic coffins that he wasn't exactly a fan of. These were more traditional, allowing him to pilot the submersible inside, where the water would then be drained and the pressure equalized. Once robotic arms took hold of the Blue Marine and stabilized it, he opened the hatch and climbed out, stretching once he was firmly on the metal grated platform leading to the elevator in the middle.
