Harry Potter had attended Hogwarts for two months, eleven days, and fourteen hours when he looked up from his Potions essay and said, "Hermione? We go to school in a castle."
His friend didn't even look up. "That's a brilliant demonstration of your thinking ability, Harry."
"No, really. I mean a castle this size has at least four hundred rooms, right? If you don't count the dormitories and common rooms and dungeons because they're so bloody cold and library, that still leaves a lot of rooms. But there's only about twenty professors, less, even, and they all teach in one room! What about the other rooms? Don't you ever think about the cool rooms that haven't been used in decades?" As he spoke, his Potions essay, half-finished, mostly quoted from an obscure book, and most definitely not his greatest work, lay forgotten on the table.
Hermione raised an eyebrow. "Are you really that against finishing your Potions essay?"
"Maybe," Harry replied, a little embarrassed. "But I also don't think we appreciate living in a real life castle all that much."
"I have to admit, I am curious..." Hermione said, trailing off. Her eyes lit up with a new wonder: there could be books in old classrooms! There could be secret passageways to help her get to her classes faster! There could even be a secret library! (The last one was a bit of a stretch, Hermione told herself, but she couldn't help but hope and imagine.)
"What are you curious about?" Ron asked, coming back from a game of exploding snap with Dean and Seamus. Unlike the other two, he hadn't started on his essay, and unlike Harry, it was much harder to guilt him into doing his homework early. Hermione had nothing on his mother.
Harry explained it to him, and Ron immediately said, "Let's go! It could be a real adventure."
"We've already had real adventures," Hermione said, remembering Halloween night with a shiver. That was already enough of an adventure for her. But Harry and Ron looked so excited, and there wasn't a rule against exploring the castle in the daytime, and Hermione was honestly very curious, so she finished with, "We can go as soon as we're done with our essays."
For a second, Ron looked mutinous, and she thought the boys might just go ahead without her. It made sense; they'd been friends for longer than she had been their friend, and Hermione sometimes (though she'd never mention it) wondered if maybe she'd never be as close as a friend to them. She didn't have much practice with friendship, after all. And she was a girl. She couldn't always be with them, and Ron and Harry were sometimes so inexplicably boyish.
She didn't want to be left behind, but she had an essay to finish and exams to study for, and another Transfiguration practical this week...
"I bet I'll be done first," Harry said to Ron, and Ron's expression turned competitive.
"Yeah, right!" he said, and ran to grab a blank sheet of parchment and a quill.
Hermione wanted to thank Harry, but her friend was already back to working on his essay. So instead, she just ducked her head and smiled. Then she dove back into her essay, determined to show Professor Snape that she wasn't just any annoying first year.
By the time all three of them had finished, Hermione had also finished another essay and studied two subjects.
"Are you sure you still want to go?" Hermione asked, dubiously looking at the time. There were only two hours until curfew. It wasn't like they could explore that much.
"C'mon," Harry replied. "We can still do something."
He and Ron determinedly left the common room, and Hermione followed along, catching up with them easily. They started in the rooms around the common room, glancing inside each one as they walked.
"We should make a map or something," Ron said as they made another turn.
Hermione agreed with a hum. It was a good idea; maybe she could even make a self-updating one! After all, she wasn't a very good artist, and although her memory was much better, she'd still love to record on the go instead of waiting until she got back. The trio talked absently about ideas for the map as they walked, mostly finding old classroom, closets, and empty rooms.
That day, they only searched the entire Gryffindor tower, and only searched the rooms they could see. Harry decided to learn more about vanishing doorways and detecting magic before he combed the tower again. Ron decided that one of the rooms looked very odd, the walls too thick to be normal, and vowed to look into it. Hermione toyed with the idea of researching the history of the tower for clues.
Three months later, Ron stumbled upon Merlin's alcove. It was magnificent: stained glass windows, floating candles lit for centuries, and the magical staffs of each of the great wizards of Merlin's day arranged in a circle inside the room. Fragile tapestries lined the walls, ancient battle scenes and old text depicted on them.
Ron sighed deeply and crossed his arms. "Hermione! I found another one!"
He heard a loud groan. "Oh, Ron!"
"It's not my fault..."
"We haven't even finished going through what we found in Hufflepuff's armory! And we're four, let me repeat that, four, rooms behind. I haven't even looked into Rowena's secret vanishing tower..."