Maia's Diary is a book, or series of books, where she writes down her visions of the future. Maia began recording a diary in the second season while pretending that her ability wasn't working anymore. Diana found a copy and foolishly told NTAC about it, leading to her being subpenaed for it. Marco created a fake version for her to hand over, which she used. Diana noticed an unusual phrase - "Mommy's bosses will be punished for betraying us."
Beginning in The New World, "all she wants to do is write in those diaries." In Gone, several of these diaries are shown at NTAC. The phrase "I'm in the circle with Tyler Downing" and doodles of butterflies help give an idea of what happened to Maia. We also find out that one day Marco will die, "in a really cool way".