Kleitman often used himself as a subject in his sleep research, particularly on sleeplessness—whether in a pitch-black cave in Kentucky, in an underwater submarine during World War II, or on a trek ...

Perhaps hoping to gain the blessing of the stone-age hunter-gatherers that dwelt along the east coast 30,000 years ago, visitors to the Baxian Caves (八仙洞) during the 1970s would grab a handful of soil ...

Located in Kinver Edge, a high heath and woodland on the border between Staffordshire and Worcestershire, the Holy Austin Rock Houses are where England's last cave dwellers are said to have lived.