Monks in northern Japan once practiced self-mummification.

Sokushinbutsu were Buddhist monks who caused their own deaths in a way that resulted in their mummification. If is believed that many monks in northern Japan attempted the act, but only between 16 and 24 mummifications have been discovered to date. 

For 1000 days, a monk would eat a special diet consisting only of nuts and seeds while taking part in a rigorous physical regimen; this stripped him of his body fat. For another 1000 days, he ate only bark and root and began drinking a poisonous tea which caused vomiting, a rapid loss of bodily fluids, and made the body too poisonous to be eaten by maggots. 

Finally, he would lock himself in a stone tomb no larger than his body where he would not budge from the lotus position. There was a bell within the tomb, and ringing it would alert everyone outside that the monk was still alive. Once the bell stopped ringing however, the tomb would be sealed. 

The other monks would wait another 1000 days and open the tomb to see if the mummification was successful. If so, the monk was immediately seen as a Buddha and put in the temple for viewing. If not, he wasn’t considered a Buddha but he was still admired for his dedication and spirit. Today, the practice has been outlawed by all Buddhist sects and the entire country of Japan.

(Source)