1. Annabeth is attracted to Piper.

Piper uses her charmspeak on all kinds of people: car salesmen, gods, monsters -- and Annabeth. Actually, Annabeth is just about the only girl in the series Piper uses charmspeak on, seen above in one of the opening moments of Mark of Athena. A book later, though, when Piper tries to use her charmspeak on the ice goddess Khione in House of Hades, it doesn't work. Because "charmspeak worked poorly if the person wasn't attracted to you."

So, what does that mean for Annabeth's reaction to Piper in Mark of Athena? Piper's charmspeak works the exact opposite of poorly on her. Piper is speaking in a murmur and isn't even targeting Annabeth, and yet Annabeth is so influenced by it she nearly drops her weapon. She may be dating Percy, but she's clearly got some unresolved feelings for her other best friend.

2. Percy was more in awe of Calypso then he let on.

Obviously, in the best love story of all time, Battle of the Labyrinth's "I Take A Permanent Vacation," Percy was in awe of Calypso. Several years later, though, when Percy and Calypso see each other for the first time in The Hidden Oracle, Percy plays it cool. Their first greeting is awkward, but every other part of the scene is mostly casual. Except for one moment.

As one of the campers tells the team the story of a scar he got, Calypso makes a simple, small gesture with her fingers toward the lemonade on the opposite end of the table, a move that would normally magically bring the lemonade to her and that Apollo only catches because he was already looking at her.

A moment after the camper leaves, when Calypso says she's lost her powers, Percy immediately cuts in and asks, "You want a drink?"

Sherman was mid-sentence when Calypso gestured toward the lemonade, so either A) Percy recognized what Calypso wanted as soon as she mentioned the loss of her powers even without seeing her gesture at the lemonade, or, perhaps more likely, B) Percy wasn't listening to Sherman and was watching Calypso the entire time, too. I don't blame him, though. I'm happy she's back, too.

3. Nico knew Hazel before "The Last Olympian."

Or at least knew of her. It's not entirely clear when Nico started visiting his sister in the Underworld after stumbling upon her while looking for Bianca, but it certainly wasn't as late into the second series as we're led to believe. In the short story, The Sword of Hades, Nico, Percy, and Thalia are summoned to the same meeting place in NYC. Nico grumbles that, before being summoned, he was in a graveyard in New Orleans. Nico has no connections to New Orleans, no reason to be near Louisiana, except for one: Hazel. Hazel is from New Orleans and is, at this point in the books, lying dead in a New Orleans graveyard. Maybe he hadn't met her just yet, but the idea that he'd looked into past children of Hades/Pluto and found Hazel's grave by this point before The Last Olympian would explain how he recognizes her in the Underworld in the first place.

4. Luke kept a diary before he was possessed.

And we don't know what happened to it. In The Diary of Luke Castellan, Luke begins keeping a diary after a new friend suggests it will help him make the right decisions in the future if he writes about them. At the end of the first entry, he says he'll keep going with it to help him sort through future choices, since he's recently found out those decisions are destined to be world-changing. After that short story ends, the diary is never mentioned, though the fact that Luke's actual world-changing decisions only appear in the PJO series, especially from The Titan's Curse forward, implies he would turn to the tool he was offered to work through them, right? When did he stop keeping the diary, or did he ever stop? Did he leave it on Mount Orthys before stepping into the coffin? Did Jason find it when he defeated Krios? I have so many questions.

5. Percy probably isn't white.

There isn't one single quote I could pull for this, but rather thirteen books worth of evidence. Sally's apartment in The Lightning Thief is in Spanish Harlem, an area that was only 7% white when TLT was written (and it is Sally's apartment, not Gabe's. Percy says as much). Percy lists the fact that a group of kids are white as a part of what's off-putting about them while he makes his way through LA in the same book. Percy says Sadie, a biracial girl, looks like what his and Annabeth's kid would look like when he meets her in the last of the Kane Chronicles crossovers. He is repeatedly described with jet black hair that is anything but straight, and dark, tanned skin that makes his green eyes pop. He is constantly branded a troublemaker by teachers, despite every scene we have of him in school proving he does nothing but listen and try to avoid actual troublemakers, all specifically because of the way he looks.

If just one of these sentences were true, it might not mean anything. The fact that there is all this and more points toward something more, though.

6. Percy and Grover had the empathy link while Percy was in Tartarus.

After Son of Neptune, it wasn't entirely clear if the magic-destroying Tiber River replica outside Camp Jupiter had done away with all of the magic linked to Percy, including the empathy link that allows Percy and Grover to connect through their emotions and psyches. This became an especially pressing issue when Percy spent an entire book in Tartarus, since the continued presence of the empathy link would mean Grover could feel Percy's emotions as he made his way through hell.

An entire series later, Grover finally reappeared and confirmed that he and Percy do still share an empathy link. Apollo mentions it very, very quickly, and mostly in reference to the reason Grover sometimes craves blueberry pancakes, but yes, Grover was still connected to Percy while he suffered Tartarus. Enjoy that mental image.

7. Percy and Hazel spent several days stuck in the bathroom together.

After Percy's fragile, post-Tartarus mental state nearly leads to him letting a sea monster kill him because he believes it to be poetic justice, he spends several days sick to the stomach from poison. At the same time, Hazel's seasickness gets worse. There are a couple maps of the Argo II, but every one seems to point toward there being only one bathroom on board the ship. It's not exactly pleasant to think about, but hopefully having company while curled over in the crew's shared bathroom helped somewhat, especially considering Percy and Hazel are very close after Son of Neptune.