Oh, I heard about something like this! It was on
QI.
TL;DW: Monks would only eat nuts and seeds for 3 years, and only bark for another 3 years, to have as little body fat as possible. They'd then drink poison that would make them vomit like crazy, expelling any bodily fluids. After that, they lock themselves up in these sarcophagus', where they slowly died and then, because of the low bodyfat, they turn into mummies. It's been outlawed, as it's basically a weird way of suicide.