Here's why:
* The rising action: Throughout the book, Greg and Rowley get increasingly frustrated with the family's cabin trip, culminating in Rowley accidentally getting locked in the basement. This sets up the conflict that needs to be resolved.
* Greg's actions: Realizing the gravity of the situation, Greg decides to risk it all and rescue Rowley. He has to overcome his own fear of the dark and the basement, as well as the challenges of navigating the cabin's layout to find the key.
* The tension: The scene where Greg searches for the key and eventually finds Rowley in the dark basement is filled with tension and suspense.
* The resolution: Greg's successful rescue of Rowley signifies the end of the main conflict and marks the turning point of the story. This leads to the falling action and ultimately, the resolution of the cabin trip ordeal.
While there are other exciting events in the book, this moment stands out as the climax because it directly addresses the central conflict and carries the most dramatic weight.