The ushnisha refers to the protuberance on the crown of the Buddha. It is one of the 32 special physical characteristics of a Buddha. In Chinese, it is called the 无见顶, which literally means ‘the top that cannot be seen’ – even by the gods and Bodhisattvas of the highest ground.
I once heard an intriguing story of how a Bodhisattva tried to look above the Buddha’s ushnisha. Tried as he might, he could not see it – even when he used his supernormal powers to rise up, he could not do so. As the ushnisha symbolises the peak of the Buddha’s unsurpassable wisdom, this is symbolic of how no one can know and see of this peak unless one becomes a Buddha personally!
【无见顶相】佛三十二相中乌瑟腻沙相中之好也（相为大相，好为小相）。佛顶上有肉块隆起为髻形，谓为乌瑟腻沙相，顶上肉髻相等，于此相中有一切人天不能见之顶点，故名无见顶相。观佛经三曰：‘佛顶肉髻生万亿光，光光相次乃至上方无量世界，诸天世人十地菩萨亦不能见。’观无量寿经曰：‘唯顶上肉髻及无见顶相不及世尊。’同天台疏曰：‘肉髻是相，无见顶是好，此相好表于极果。今作因人，故不及佛。’楞严经七曰：‘若有宿习不能灭除，汝教是人一心诵我佛顶光明摩诃萨怛多般怛罗无上神咒，斯是如来无见顶相无为心，佛从顶发辉，坐宝莲华所说心咒。’ [丁福保佛学大词典]
If not one can see how did they know? did the Buddha tell them himself?
Hah! I’ll be happy to know the scriptural source of this charming story to have the details too! Amituofo
😉