In 1972, French geologist and speleologist Michel Siffre descended 440 feet into a cave near Midland, Texas, leaving behind every conventional marker of time. There were no clocks, no sunlight, and no connection to the outside world. For six months, Siffre lived in complete isolation as part of a radical scientific experiment designed to understand how humans perceive time when all external cues are removed. At first, he attempted to follow a routine. But as the weeks passed, his internal sense