Whatever Deku was expecting when he and Eraserhead made it to Musutafu, it wasn't the giant crater they found. Piles of wreckage spilled over its edges, and dozens of Heroes and emergency responders were at the edges of the site, working through the wreckage to rescue civilians and evacuate the area. He noticed Red Riot a little ways off, directing some of the paramedics on focus points, and made his way over to him. As soon as Red Riot saw them, he moved toward them to meet them partway, calling out a quick greeting. "Hey guys, rescue's in full swing. Still not certain of the numbers at this point, but it's bad. Ground Zero needs the both of you with him asap. I don't know the full details, but the villains he was up against are apparently connected to some case you guys have all been working on or somethin. He's at the other side, in the basement of a building that apparently connects to a bunch of tunnels and basements all across town. It'll be the building with the tape and dividers surrounding it." He smacks Deku on the back before running off to continue the rescue work, leaving Deku and Eraserhead a little confused- they haven't been working on any cases together recently, so what case can he be talking about? They look at each other for a half-second, equally confused looks on their faces, before they start their way across the wreckage.
They make it to the building in question quickly, where they are greeted by a couple of police officers who warn them to steel themselves before they go inside but wave them in with little fanfare other to hand them hazmat suits. They don't put them on immediately, in case it gets in the way of a fight, but they prepare themselves for the eventuality of wearing them when the need becomes apparent. The lights inside are flickering, half burnt-out, and a layer of dust covers everything, shaken loose from the explosion. It appears to be a normal, single-story home inside and out, which doesn't answer any of the questions running through their minds. They head to the basement stairway, made obvious by the wide-open door, and down the rickety wooden stairs to a plain concrete basement with a single exposed bulb providing light. From the angle the stairs are at as they head down, it doesn't look abnormal in any way, just a plain square room with a washer and dryer, but once they're standing on the concrete floor and looking around, they can see what makes this no normal basement: a giant, round, metal archway is recessed in one wall, standing wide open and ominous. There's some kind of archaic mechanism to one side, similar to a steering wheel, in the same metal color. The doorway is thick, at least a foot deep, but no door is in sight.
On the other side of the doorway is a long dark hallway, lit only by a string of small lights stapled to either side, similar to large christmas lights. It looks old and well-constructed, like something that's been around for hundreds of years. Ground Zero isn't in sight, and there's nothing else in the room, so it's apparent they're expected to follow this tunnel to find their colleague. They head down the narrow passage for a couple hundred meters as it twists and turns and angles slightly downhill but doesn't break off into separate branches, until they see a warm glow ahead. They prepare for an attack as the creep forward the last few meters, only to find themselves in some kind of underground amphitheater. There are dozens of tunnels just like the one they just came through lining the walls, and the room's walls and floor slope up and down so much the room is nearly spherical. There are dozens of stairs heading down toward the center of the room, where a large recess cuts straight into the floor a few feet. In the recess, which is surrounded by a rusty domed fence, they can see a few people in hazmat suits walking around the recessed area and precariously over the uneven ground: some emergency personnel, Ground Zero, and a few sidekicks and interns Deku vaguely remembers have analysis-based quirks.
It's the smell that hits their noses first as they enter the room, foul and murky and overpowering now that they're out of the tunnel, with a hint of sweet that is so inherently rot they reflexively wrinkle their noses at the gut-churning odour. Deku looks over into Eraserhead's eyes, hoping against hope he's come to a different conclusion than the one that's swirling in his mind, but it's obvious from the way he's looking back at him that he hasn't, so they don their hazmat suits quickly. Deku and Eraserhead trot down the long staircase toward the center, their worst fears confirmed as they move closer: the recess is a pit, and the ground of it is covered in rotting corpses.
The bodies are in various stages of decay, a few barely more than a few days old and a few so long-dead they're more sludge than body. It's horrific that some of the people who arrived before them are walking on dead bodies, but it's an unfortunately needed circumstance in order to bring some of the newest bodies out of the pit for identification and examination. Ground Zero noticed them when they entered, but he waits for them to walk down the stairs so they have time to process what they're seeing before he growls out their names in greeting and plunges into a quick recount of the events up to this point. "Was on the way back home from the old hag's place when I saw some movement in one of the alleys nearby. Couple lowlifes had accosted a chick and were trying to shove her through the exterior access to the basement of a house a couple blocks away from the one you guys came in from. When I got closer to investigate, they spooked and took off. Followed them, but when it became apparent they weren't getting away, one of them stayed back to fight. Had some kind of forced-activation enhancement Quirk that he hit me with, forced my Quirk to activate and enhanced the explosion, well... You saw the results outside. Explosion killed the enhancement guy outright, but the other guy made it out. Followed him into the house, through the tunnel, but lost him here since he had a pretty good headstart and I was running on fumes after that burst. No idea which tunnel he took to get out." He looks out over the room for a moment, a deep sadness permeating his silhouette, before he continues, "After identifying a few of the victims, we've been able to match most of them as suspected victims to two separate cases. One is the trafficking ring Deku and I investigated and raided a couple months back, and the other is a string of murders you've been apparently keeping tabs on, Eraser. The bodies show the hallmarks of your killer, and several of them are ones that were missing from the roster you and I constructed from the trafficking victims' accounts, Deku. I don't know how the fuck something so big has stayed hidden for so long."
Deku's mind was reeling with the information; a group of traffickers, connected to one or more perpetrators in a long series of murders, and a giant underground colliseum? How do any of those relate to one another? And how did they know this place existed? Is this yakuza, maybe? Or some sort of cult? It was assumed people were being trafficked, but what if that trafficking ring wasn't actually selling their captives off? Then what would be the purpose of keeping them alive for so long if they're only going to kill them and discard them here later? And why discard some of the victims where they can be found and labeled as murders instead of dumping them here so they're less likely to get caught?
"My guess is experimentation or Quirk breeding," Eraserhead cut into Deku's mumbled thought process. "It would explain the time lapses. Perhaps the murder victims were deemed unusable and discarded?" Ground Zero nods in agreement; looks like he came to the same conclusion, then. "I'll take a look at a few of the bodies to confirm it's not a copy-cat so we can definitively rule them linked. Has anyone explored the rest of these tunnels yet?"
Ground Zero nodded shortly. "Yeah, we've sent groups of two to check out every tunnel as soon as we got enough people on-site. It's taking some time since we don't know what they're going to find, but the first groups should be back soon. They're set to report to me as soon as they are. Deku, go check in with Nakamura over at the cage entrance so you can get a full rundown of what we know so far. We're going to need your analysis before we make any decisions where to go from here." With that, the three separate to look into the case. Between their intimate knowledge of both cases, Deku's ability to analyze better than just about anyone other than maybe Nezu, and the detectives, they're confident they should have at least a couple of leads before the end of the night.