Such practice is quite common within east asia. It is called 'sokushinbutsu' in Japanese, kanji of which translated into 即身仏, which means 肉身佛 in Chinese, literally 'flesh body buddha'. From wikipedia, monk Hui Neng, dating back to 638 A.D. tang dynasty, seems to be believed the first of such example.<p><a href="https://en.wikipedia.org/wiki/Huineng" rel="nofollow">https://en.wikipedia.org/wiki/Huineng</a><p>I cannot say I understand how this is explained from Buddhism within, because I am not practitioner. But if such thing happens, it is usually viewed as some kind of wonder/miracle by the locals, and also means the monk has ascended to be Buddha.