Acrophobia: An extreme or irrational fear or phobia of heights.
When Thalia was a little girl, travelling with Luke, she fell through the ceiling of a warehouse. The wiring of the warehouse was the only thing that kept her from kissing the ground face first. The wiring made her stop mere centimeters from the dirty floor.
Luke tried to get to her as soon as he saw her fall, but gravity was faster. He nearly went tumbling down with her, but caught himself in time.
When the cyclops caught them, he suspended them in the air. They could have escaped sooner if Thalia's fear of heights did not get in the way. She had her concealed knife in her hand, but could not get herslef to cut the rope around them. It was the only thing keeping them from crashing to the ground.
The more Luke told her that they would not get hurt falling from the height they were suspended from, the more Thalia saw the warehouse floor. It wasn't until she heard Annabeth's screams for grover that she cut the rope.
And then, when she died, Zeus just had to turn her into a tree. A tree. The tallest structure around for miles...
