Tony was the one who found the dungeon.
When Agent Coulson asked him to round up a few Avengers and raid a laboratory owned by Advanced Idea Mechanics, a.k.a Hydra, he hadn't mentioned any hidden rooms.
Tony briefed the Captain, Bucky, and Nat and on the superintelligence A.I.M. was trying to create and how many scientists and guards they should expect. They left at sundown.
By 10 PM, Tony had called and told a relieved Coulson that A.I.M. had yet to succeed in their endeavor. Cap and Bucky were outside, Cap answering questions from the press and Bucky avoiding the memories the labs triggered.
Inside, Nat was taking her time making sure she hadn't missed any hard drives worth cracking. Tony tried to speed up the process by looking through the rest of the facility while she worked. There was only one story above ground, a rather plain collection of offices and sterile labs. Underground, there were exam rooms that looked more like torture chambers, but they hadn't found any "patients", only experiments.
Tony walked from room to room, checking for anything he or Nat might want to inventory. He paced the hallway three times before he realized what was nagging him. Several of the exam rooms included two-way mirrors that connected to observation areas. Amongst all the steel and plastic of one of the observation rooms was a wooden bookshelf.
The small, plain shelf held a collection of medical journals and textbooks, but none of them seemed particularly relevant to the experiments that had been conducted in the facility. Tony opened his mask as he thumbed through one of the volumes. As he put it back on the shelf, he felt a breeze on his cheek. He looked around but there were no vents nearby, nothing to send a wisp of cold wind his way. And then he knew.
Iron Man grabbed the edges of the bookshelf and ripped it away from the wall, revealing a brick-lined wall and a set of steel stairs. He put his mask down and padded down the stairs, opening the coms as he descended.
"There's something else down here, Nat. Wasn't on the blueprints."
"What is it?"
"Hold on." Tony reached the bottom the stairs. There were no guards or anyone else in sight, just another row of doors identical to the exam room doors above. But these rooms weren't for examinations.
The first room held a huge bed with plush pillows and a mirror on the ceiling. The walls of the second room were lined with whips, floggers, and canes. There was a suspension contraption in the middle with enough cuffs to hold four people in place. The third room did look like an exam room, except for the storage containers filled with a variety of dildos and plugs larger than most adult stores. Another room had an inch of water on the floor and cattle prods hanging on hooks. There were twenty rooms in all.
At the end of the hall, there was a room larger than any of the others filled with bunk beds. It reminded Tony of military barracks but, instead of soldiers, there were naked women with their ankles and wrists chained to the beds.
Tony sighed. "Nat, I'm gonna need your help down here."