French scientist Michel Siffre spent two months alone in a cave with no clock, no calendar, and no sunlight — and when his team finally told him the experiment was over, he thought he still had nearly a month left underground (opens in new tab)

When Michel Siffre’s support team called down to tell him the experiment was finished, he did not believe them. By his own reckoning it was around the twentieth of August. The real date was the fourteenth of September. He had spent about two months alone underground, and he thought he had nearly a month still […]