Deep

Characters: Polar Tang, Heart Pirates. Rating: K+. Warnings: None

For many people, the idea that a submarine had a maximum diving depth was something that never crossed their minds. Even the Heart Pirates barely gave it a second thought, used to cruising along in the twilight zone of the ocean, occasionally going deeper if they needed to.

The Polar Tang hadn't come with a user manual. She probably had one, somewhere – and sometimes Law would worry about that, when he remembered there was confidential information about his ship in the hands of the World Government, if they ever thought to look – but it hadn't been anywhere on here when they'd searched, so everything was a case of trial and error.

They hadn't truly trialled her maximum depth. The first time they'd gone deep had been an accident, three teenagers and a mink cub going into a knee-jerk reaction at a pink flamingo on the horizon. They'd dived through the currents until they'd hit… something. Something fast and furious that wrested all control away from them and sent them hurtling towards the sea floor, only to spit them out at the last possible second.

Torn between the terror of Doflamingo and the terror of the event, the four had ended up huddled together on the floor of the control room, not knowing if they should try to go back up or if they were safe where they were. The Tang's readings proclaimed that they were down at six thousand metres, the first time they'd gone into a four digit number at all. They hadn't thought her scale went down so far, but now, as they watched, the graph rolled, exposing further numbers below.

If the scale went that far, then they were okay, Law eventually determined, extracting himself from the shaking pile to inspect the instruments more carefully. The lower number was still concealed from view, but the pressure gauge alongside told a more worrying story. Law had been vaguely aware that the deeper underwater you went, the higher the pressure would be. The number glaring at him on the screen was far, far higher than he'd even imagined, and for a moment he feared that the submarine would crumple in on itself.

She didn't. There were no groans or creaks of metal, just the humming of the engines as she propelled herself along. The temperature was higher and the humming was louder, but there were no other signs of the additional strain she was under, and for the first time the question struck Law.

Just how deep could she go?

He didn't experiment. First, they had to find a way to get back up to their usual diving levels before their air suffered, and then it would be time for research. What had sucked them down so fast, was it the only way down to their current depths, and how could they utilise it?

Feeling the Polar Tang staying true in such pressure wiped away all of Law's fear, replacing it with curiosity. If they could get to these depths intentionally, Doflamingo would never be able to reach them, not even with his strings. Had they stumbled across the best hiding place in the ocean?

The answer, it transpired after they found a current that carried them skyward and gently deposited them back in the twilight zone, was yes. The Downflow only existed in certain areas, but with additional engine power, the Tang could dive to the same depth under her own power – a far safer, less hair-raising ride – and so six thousand metres below the surface became their new usual haunt.

It wasn't without its dangers, of course. True sea monsters lived at those depths, some scared off by the bright lights of the Tang blinding them but others lured in, and the pirates quickly learnt to use the Polar Tang's built-in weaponry when necessary.

The question didn't come up again for a long time, until the now adult crew, swollen in number to a fully fledged pirate crew with a reputation to match, departed Sabaody for the last time in their journey forwards.

Fishman Island was ten thousand metres below sea level. They'd become fond of cruising at depths of six or seven thousand, but ten thousand was unexplored territory.

"Can she do it?" someone asked nervously as they began the initial descent. "I mean, we could get her coated, to be safe, right?"

"If she can't do it, no-one can," Shachi had said firmly, resting a hand lovingly on the pipes. "We've got the best ship in the whole world, just you watch."

"Coating only works for sailboats," Penguin had added. "Just because she has a sail doesn't make her one of those."

The decision had been to keep going, deeper and deeper through the sunlight zone and into the twilight zone. The Downflow in the twilight zone was the obvious route to take, but the Heart Pirates had become a crew well-versed in hiding. Taking the obvious route meant lining themselves up for potential traps, so with enough research (mainly on Bepo's half, but they'd all chipped in), they'd found and plotted an alternative route to the dark levels, using a smaller version of the current the other side of the archipelago, which would bring them down to the deep sea earlier than most ships. Not having to rely on a bubble for their air supply, they could afford to go deeper earlier.

They'd concluded that using a Downflow (if not the Downflow) was for the best. That way, they could all but cut their engines, using enough power just to keep them on course and thereby saving fuel. After all, who knew what would be waiting for them when they surfaced the other side of the Red Line? They couldn't afford to use up most of their fuel supplies.

Still, that didn't stop more than a few members of the crew gulping in trepidation as the current loomed in front of them, impossibly large and fast as it sucked them in violently, catapulting them into the murky depths far below. Their instruments reported a familiar depth of seven thousand metres when they were spat out, engines roaring to life to control their exit as best the Polar Tang could, and it was then their adventure truly began.

For other crews, snuggled up safely inside their coated sailboats, the hardest part would be over by that point. If they'd survived being tossed around in a violent current surrounded by enough rocks to wreck a fleet, the last three thousand feet were relatively plain sailing, so long as they watched out for the local sea life.

For the Heart Pirates – and the Polar Tang herself – this was where it started. Slowly, they began to dive, keeping a careful eye on both their depth and the surrounding pressure. Fending off the sea monsters was second nature to them, helped by their kairoseki hull and specialised weaponry, which was undoubtedly a good thing, as nerves began to rise the lower they went.

Any moment, they feared they'd hear an out of place creak, or a groan as the metal shell of their beloved submarine began to strain under the ever-increasing pressure. None of them, not even the captain, were at ease as they descended slowly, hoping that keeping the speed gentle would help keep the pressure inside and out even.

When their sonar finally reported Fishman Island, their instruments loudly declaring that they'd reached ten thousand metres below sea level, nervous breaths were shakily released. They'd always known the Polar Tang was special, but as she cruised the last stretch at ten thousand metres below sea level, louder engines and a stuffy heat the only signs that she was battling through pressure a thousand times greater than the surface of the sea, they realised just how special she was.

Okay, so throwing away real life physics here, where submarines don't seem to be able to handle more than 1000m below sea level, I present the question: how deep can the Polar Tang go? The Shark Submerge can go to 5000m below, and we've seen (in the anime) the Polar Tang going through an underwater volcano field, which seems to be around 7000m deep, if we go from the Strawhat's own depth when they encountered one. However, Fishman Island is 10000m below. I don't see them coating her, because coating a submarine seems a bit silly, in all honesty. Besides, if the Shark Submerge, which was built by Franky out of what I suspect is scrap metal, can get to 5000m, then a professional-grade submarine must be capable of going far deeper.

As for their odd route to Fishman Island, underwater is their playground. If anyone can find a different route down to the bottom, I'm willing to bet it would be them.

Thanks for reading!
Tsari