In film theory, an "index" is a sign that has a direct, causal connection to its referent. In The Happening , the "index of the happening" refers to the visual cues that signal the onset of the invisible toxin.

Leo looked at his hands. They were the same hands that had touched the watch two years ago. The file had recorded that touch. It had always recorded it. The index didn’t predict the future. It was the future, written down before the ink dried, because in the architecture of the universe, everything had already happened. They were just living the table of contents backward.

By reducing complex flood events to a binary "happening," it allows for massive datasets (spanning 1985–2021) to be processed efficiently.

“Carefully go through your day. Note everything that does not leave a trace.” — from the first entry of the Index

Thus, the "Index of the Happening" is not a single file. It is a constellation of sources: photographic negatives, handwritten scores, receipts for paint, legal waivers, and VHS tapes.