Michel Augustin Francis Siffre (3 January 1939 – 25 August 2024) was a French underground explorer, adventurer and scientist.

Inspired by the space race, he explored how humans experience time by spending two months cloistered 130 meters below the surface[3] in the abyss of Scarasson (Punta Marguareis) without time cues on a glacier, from July 1962.

In 1972, Siffre went back underground for a six-month stay in a Midnight Cave in Texas.

[5] He found that without time cues, several people including himself adjusted to a 48-hour rather than a 24-hour cycle.

[citation needed] His final excursion in a cave ran from November 1999 to February 2000; he celebrated the New Year there, but missed the actual date by four days.