Thalia's Pine Tree

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

History
Five years before Percy got to Camp Half-Blood, Thalia, Annabeth, Luke, and Grover were traveling to Camp Half-Blood. Thalia was the demigod daughter of Zeus, Luke of Hermes and Annabeth of Athena.

Thalia's birth was considered an outrage considering the fact that the Big Three (Zeus, Poseidon and Hades) made an oath to never sire demigod children. Hades was furious and sent a whole army of monsters from the Underworld after them. Grover told Percy in The Lightning Thief that he made a couple of wrong turns (one of the worst towards a Cyclopes' lair in Brooklyn) and reached Camp Half-Blood too late. The monsters were only a few yards behind them. But before they could reach the camp's safety (inside the camp's magical borders at the entrance), the monsters attacked.

Thalia told Luke, Annabeth, and Grover to run while she confronted the monsters. At Thalia's request, the others ran for the borders of Camp Half-Blood. During the battle, Thalia was mortally wounded. As she lay dying, Zeus took pity on his daughter and turned her into a pine tree to preserve her soul. From that day on, her pine tree represented the camp's border, and gave the hill it's name. It fended off all monsters and mortals.

The only way for a monster to get in is to be summoned inside the the camp or to be invited into the camp as shown in The Sea of Monsters when Annabeth allowed Tyson, Percy's cyclops half brother, into the camp just so he could help Percy defeat the bronze Colchis bulls that were invading Camp Half-Blood.

The Sea of Monsters
Percy, Annabeth, and Tyson run off to go looking for the Golden Fleece, the only cure for Thalia's tree after it had been poisoned by Luke. Coincidentally, Grover found it while looking for the God of the wild, 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 guardian yet), Thalia was expelled from the tree having only aged at the rate of the tree. This however this was Kronos' plan all along to give him another chance at controlling 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.