Northern Japan between the 11th and 19th century.
It is believed that many hundreds of monks tried, but only 24 such mummifications have been discovered to date.
1,000 days of eating a special diet consisting only of nuts and seeds, while taking part in a regimen of rigorous physical activity that stripped them of their body fat. They then ate only bark and roots for another thousand days and began drinking a poisonous tea made from the sap of the Urushi tree, normally used to lacquer bowls
This caused vomiting and a rapid loss of bodily fluids, and most importantly, it made the body too poisonous to be eaten by maggots. Finally, a self-mummifying monk would lock himself in a stone tomb barely larger than his body, where he would not move from the lotus position. His only connection to the outside world was an air tube and a bell. Each day he rang a bell to let those outside know that he was still alive.
When the bell stopped ringing, the tube was removed and the tomb sealed. After the tomb was sealed, the other monks in the temple would wait another 1,000 days, and open the tomb to see if the mummification was successful. If the monk had been successfully mummified, they were immediately seen as a Buddha and put in the temple for viewing. Usually, though, there was just a decomposed body
There's no doubt now that they are dead but corpses should always lie down.
And that space of just the bell ringing, and then not ringing and then the wait, and then the answer.