Michel Siffre (born 3 January 1939) is a French underground explorer, adventurer and scientist. He was born in Nice, where he spent his childhood.

He received a postgraduate degree at the Sorbonne six months after completing his baccalauréat. He founded the French Institute of Speleology^{[importance?]} (Institut français de spéléologie) in 1962^[1] (not to be confused with the French Federation of Speleology).

Inspired by the space race, he explored how humans experience time by spending two months cloistered 130 meters below the surface^[2] in the abyss of Scarasson (Punta Marguareis) without time cues on a glacier, from July 1962.^[3] He then organized several similar underground experiments for other speleologists. In 1972, Siffre went back underground for a six-month stay in a cave in Texas. He found that without time cues, several people including himself adjusted to a 48-hour rather than a 24-hour cycle.^[4] The notes of his experiments were used by NASA. Several astronauts reported experiences similar to those experienced in underground experiments such as loss of short-term memory to being isolated from external time references.

• Hors du temps. L'expérience du 16 juillet 1962 au fond du gouffre de Scarasson par celui qui l'a vécue, Julliard, 1963
• Des merveilles sous la terre, Hachette, cop. 1976
• Stalactites, stalagmites, cop. 1984
• L'or des gouffres: découvertes dans les jungles mayas, Flammarion, 1979
• Dans les abîmes de la terre, Flammarion, 1975
• La France des grottes et cavernes, Privat, 1999
• A la recherche de l'art des cavernes du pays Maya, A. Lefeuvre, 1979
• Découvertes dans les grottes mayas, Arthaud, 1993
• Beyond Time, Translated by Herma Briffault, McGraw-Hill, New York, 1964