It was a person free-running in a cave. Here's one article about it [0] to get you started, and here's another [1] of multiple 9-day studies showing the fairly immediate shift to 24.5 - 25 hour cycles (we're actually Martians?).

[0] https://howandwhys.com/michel-siffre-time-experiment-body-sl...

[1] https://www.researchgate.net/publication/339110740_Living_Wi...