Thalia's tree

Thalia's tree is a pine tree which marks the official border of Camp Half-Blood. Percy describes it as a White House Christmas tree-sized pine tree.

History of Thalia's tree
Five years before Percy got to Camp Half-Blood, Thalia, Annabeth, Luke, and Grover were traveling to Camp Half-Blood. Thalia was the daughter of Zeus. That was considered an outrage considering the fact that the Big Three made an oath to never have kids with a mortal woman again. Hades was furious and sent hundreds of monsters after them. Meanwhile Grover, as said in the book, made a wrong turn and reached Camp Half-Blood late. The monsters were only a few yards behind them. Before they reached the camp's safety, the monsters attacked.

Thalia told Luke, Annabeth, and Grover to run while she attacked the monsters. At Thalia's request the others ran for it. During the battle, Thalia was mortally wounded. Zeus pitied his daughter and turned her into a tree so that her soul would not go to Hades. From that day on, that represented the camp's border. It fended off all monsters and mortals. The only way for a monster to get in is to be summoned in.

Rebirth of Thalia
In The Sea of Monsters, Percy, Annabeth, and Tyson go looking for the Golden Fleece, the only cure for Thalia's tree. Coincidentally, Grover found it while looking for the Great God Pan. After their adventure in The Sea of Monsters, they put the Golden Fleece on Thalia's tree. Immediately the tree begins to heal and expel the poison. Several days (or weeks) later, while Annabeth was on watch (because there was no monster guardian yet), Thalia was expeled from the tree. This however was Kronos' plan all along to give him another chance at controling the prophecy.

The pine tree is still present, except now it uses the Golden Fleece for energy and is being guarded by a dragon named Peleus.