*??? 

*Evan/My POV

After being underwater for like forever, Kirov came up to me with news. 

"We've arrived at our destination Comrade." Kirov replied. 

"Do we need to switch into smaller expedition vessels if we're exploring underwater ruins or something?" I asked. 

"We can just walk off from the ship. There's no need for that." Kirov replied. 

"Aren't we still underwater?" I pointed out. 

"To be precise, this is a unique space that resembles a Mirror Sea, except underwater. The specifics are a bit more complicated, but you'll see once you get out." Kirov explained. 

"An underwater Mirror Sea? What?" I asked. 

"Comrade, welcome to the Northern Parliament's Arcana Sanctum!" Belorussiya exclaimed.

*Timeskip

*Arcana Sanctum 

After exiting the hatch, I was surprised to discover that there was no water pressure. A wide, spacious area stretched out in front of my eyes. When I looked down at my feet, I could see a layer of calm ocean water coursing beneath me. Lifting my head, I could see that the dome of stars that should've been above me was now replaced by the expansive sea. Around me, icebergs cut neatly into titanic slabs floated overhead serenely, and two tracks swirled overhead as they spiraled toward the surface. Not even in my wildest imagination would I have been able to envisage such a surreal dreamscape. At first, I expected this "Arcana Sanctum" to be some sort of naval base. Then, I thought it was some abandoned underwater facility. But this... could something so spectacular truly exist in this world...? 

"Wow." Le Triomphant looked at the scenery with awe. 

"It's been some time since I last came here. A marvelous sight to behold, as always." Kirov remarked. 

"You all came here before?" I asked. 

"We sure did Comrade." Kirov replied 

"Heh. You all look completely speechless, Comrades." Tallinn chuckled. 

"Ehehe~ I was just like that the first time I came here... It takes a while to let everything sink in, you know~" Murmansk giggled. 

"Comrades, we need to get moving. You'll find even more wondrous things deeper inside." Belorussiya reminded before turning to me.

"Take this submarine as your command ship for now. The rest of us will spread out and enter an escort formation as we head through the first Gate." Belorussiya instructed and I nodded. 

"The icebergs floating here must be hundreds of meters high." Lady Forbin remarked. 

"From 50 to 200 meters, to be precise, and their surfaces are flat and smooth. As we all know, most of an iceberg's actual mass is hidden under the surface — in theory. Based on our observations, however, the icebergs here do not have substantial underwater structures. Rather than "floating," they seem to be suspended within the water." Tallinn explained. 

"Are these icebergs all a part of the Mirror Sea...?" I asked. 

"That's not all! The Mirror Sea periodically produces Siren defense modules around these parts! These modules are rather dated, but we can't just ignore them either. Per our orders, we have to return here periodically to sweep any enemies that might spawn, and these icebergs sometimes get split apart by stray fire... But... after we leave and come back, the icebergs simply reform as if nothing ever happened!" Gromky replied. 

"The ice is capable of regrowing...?" Le Triomphant asked. 

"That's right~! Pretty spooky stuff, right? So that's why we still have to carry out lots of experiments. When the icebergs are lightly damaged, they regenerate into their original forms. If we blast 'em to pieces, the fragments disappear, but then an identical iceberg will float up to take its place soon afterwards!" Murmansk replied. 

"So far, the Mirror Seas we've encountered can be broken up into two categories: one, arsenals for making Pieces as well as mass-producing ships; and two, experimental research facilities for creating unfathomable weapons. Of all the Mirror Seas discovered by mankind, this may be the first that does not fall into one of those categories. We've conducted extensive long-term research on this place, gathering as much data as possible and analyzing any samples that we can get our hands on. However, we still have no idea what the purpose of setting up a Mirror Sea like this might be - a hidden underwater facility that seems only to preserve a landscape. Finally, we made a hypothesis... Comrade, what if Mirror Seas were not originally meant to be tools of war, but were designed to be tools of stealth and evasion?" Kirov asked me. 

"Tools for stealth and evasion?" Lady Forbin asked. 

"What do you mean by that?" I asked. 

"Generally speaking, Mirror Seas are unrecognizable from the outside until you've actually entered them. The single thing that all Mirror Seas have in common is their concealment." Tallinn explained and I nodded in agreement. 

"On the battlefield, the Sirens often directly convert a portion of the ocean into a Mirror Sea, suggesting that whatever device they use to do this must be portable. As for all the attacks we've seen both from and inside the Mirror Seas, those might just be products of the equipment stored inside. As someone who formerly stayed in Iron Blood, I can attest that they started researching Mirror Seas very early on." Tallinn continued. 

"Le Capitaine. Do you know all of this?" Le Triomphant asked me. I looked at her and shook my head.

"You know things are getting serious when even Sir Evan doesn't know about this." Lady Forbin remarked. 

"The impression I got was that initially, Iron Blood wanted to monopolize the fruits of their research on Siren technology, and refused to join the other major powers in the Azur Lane for that reason. At first, Iron Blood was not a primary target for the first wave of Siren attacks. Ironically, their meddling into Siren technology stirred the sleeping beast, so to speak, and they suffered devastating losses as a result." Kirov stated. 

"Mhm. Iron Blood has always been obsessed with Siren technology, through and through. Even before I left, they already managed to disassemble components of the Mirror Sea. I assume they're still ironing out technical challenges for the various modules, but I don't know how much progress they've made." Tallinn guessed before dropping her cigarette on the floor and stepping on it. 

"What I'm taking away from all this is that we have to distinguish between the Mirror Seas we encounter in battle, and the actual concept of a Mirror Sea, right? An actual Mirror Sea is just a technological device designed for absolute privacy and portability, whereas what we see in practice is like an upgraded, personalized version of that?" I tried to summarize. 

"That sounds about right. Though Mirror Seas can be a pain in the ass when used offensively, we are able to destroy them directly as long as we have the numbers. But what if Mirror Seas are used to hide away places just like this one? We may never find those places... or know how many there are." Tallinn agreed with me before stating another fact. 

"That does sound plausible. Perhaps the Sirens' main base, which has never been pinpointed so far, is hidden somewhere in the depths of the ocean in a similar form?" I guessed. 

"We reached the same conclusion, but because we've never found a second Mirror Sea that shares the same conditions as this one, we need a lot more evidence before making that logical leap." Belorussiya concluded.

"Wait, if the Sirens are hiding underwater in well-concealed Mirror Seas, how was the Northern Parliament able to seize control of this 'Arcana Sanctum'? I can't help but feel that there's more to this story that they're not telling me..." I thought.