The problem the assembled deities faced was not how to force a door. It was how to persuade a goddess to open it herself. Their solution was to make her curious about her own reflection, and the object they made for the purpose is now the senior item of the Japanese imperial regalia.
Made to a specification
The Kojiki describes the manufacture with unusual attention to procedure. Iron is taken from a mine on the heavenly mountain; a smith deity, Amatsumara, is sought out; and Ishikoridome — a female deity whose name has been read as relating to stone-cutting and mould-making, which is what casting a bronze mirror actually requires — is commissioned to make the mirror. Jewels are strung by another specialist, a sakaki tree is uprooted whole, and the mirror and beads are hung from its branches with cloth streamers.
The Nihon Shoki adds a detail the shrine tradition preserves: a first attempt was rejected as flawed, and a second was made. The rejected mirror is identified with the object at Hinokuma Shrine in Wakayama, which is the sort of claim that shrines make and which is unfalsifiable and old.
The name means the eight-span mirror. As elsewhere in this material, eight is a way of saying large rather than a measurement, though some readings take it as a reference to an eight-lobed shape — and eight-lobed bronze mirrors do exist in the Japanese archaeological record.
Used as an argument
When the rock door opens a crack and Amaterasu asks why the deities are laughing in the dark, the answer she is given is that a greater deity has arrived. The mirror is pushed forward at that moment.
What she sees is her own light, and she does not recognise it. Leaning out to look more closely at the rival, she is seized and pulled clear, and the door is roped shut behind her.
It is worth being precise about what that scene claims. The sun is drawn out of hiding by being shown herself and mistaking herself for someone else. A mirror in this tradition does not flatter, decorate or reveal a truth to its owner; it presents an image so convincing that a deity is deceived by it. That is the theological content of every mirror in every Japanese shrine.
The instruction that founded Ise
When Ninigi is sent down, Amaterasu hands him the mirror with an order recorded in both chronicles: to treat it as her own spirit, and to revere it in the same house and with the same reverence he would show her.
That sentence does two things. It makes the mirror a shintai — a body in which a deity resides, rather than an image of one — and it establishes the principle on which Japanese shrine practice runs. What is in the innermost chamber of a Shinto shrine is not a statue. It is an object: a mirror most often, sometimes a sword, a stone, a jewel or a scroll, and at some shrines a mountain that is not in a building at all. It is not displayed. Priests of most shrines have never seen their own.
The mirror was kept in the palace until the reign of Emperor Sujin, when — the Nihon Shoki says — the sovereign found cohabiting with it insupportable and had it moved out, after which the princess Yamatohime carried it on a long circuit of provinces before the deity indicated Ise. A replica was made for the palace, and that replica is the object in the imperial sanctuary today.
Fires, replicas and the honest gap in the record
The palace mirror's history is a record of fires. It is reported damaged or destroyed in blazes in the tenth and eleventh centuries, most notably in 960 and 1005, and there are accounts of fragments being gathered from ashes and of the object being remade or re-enshrined. Medieval sources disagree about how much of anything survived.
The Ise mirror is a separate object with a separate custodial line, and Ise's practice of rebuilding the entire shrine complex every twenty years, moving the deity to the new sanctuary at night under cover, means the mirror has been ceremonially transferred more than sixty times without anyone reporting on its condition.
Any confident statement about what the object is, therefore, is a statement about which claim you have chosen to believe. This is one of the few places on this site where the honest answer is that the physical history is unrecoverable by design.
Not carried at the enthronement
At the accession ceremonies, the sword and the jewel are presented as wrapped bundles. The mirror is not. It stays at Ise; the palace replica stays in the palace sanctuary; and the new emperor's first duty towards it is to report his accession by envoy rather than to receive it.
Of the three regalia, then, the one described as containing the sun goddess's own spirit is the one that does not attend. That is not an oversight. An object that is a deity does not travel to witness a human ceremony.
Mirrors everywhere else
Bronze mirrors are among the most abundant prestige objects in Japanese archaeology. Kofun-period tombs yield them in numbers, including large numbers of Chinese imports and the distinctive Japanese-made triangular-rimmed mirrors with deity-and-beast decoration that have generated one of the longest-running disputes in Japanese archaeology about where they were made and what their distribution means. Whatever the mirror of the myth was, mirrors as buried marks of authority are hard fact for the period.
In shrine practice today a round mirror on a stand is visible at the front of a great many halls — that one is generally not the shintai but a stand-in, placed where worshippers can see it. The distinction is rarely explained on site and is worth knowing before you assume you are looking at anything of consequence.
Sacred geometry and game inventories
The regalia set is a standard fixture of Japanese games and anime, from Persona and Nioh to Ōkami to countless gacha rosters, usually as three collectible artifacts with elemental properties. The mirror is generally cast as the defensive or revealing item, which is a reasonable extrapolation and not what the chronicles say.
The mirror is also, independently, one of the most heavily loaded objects in world symbol traditions — as a device for truth, for vanity, for doubling and for passage — and Japanese usage sits oddly within that. Here it is neither a truth-teller nor a trap. It is a container.
Why a mirror rather than a statue?
Because early kami worship had no images at all. Shrines began as marked-off ground, a rope, a rock or a tree, with the deity summoned to a temporary seat and then departing; permanent buildings came later, and figurative representation of kami is essentially a Buddhist import, appearing only after temple sculpture had made statues familiar. In that context a mirror is close to ideal: it is a valuable manufactured object, it is associated with authority through the tomb finds, it holds light, and it shows something without depicting anything. What it contains changes with what stands in front of it, which is a very exact match for a deity understood as present rather than portrayed.
Does the mirror have an inscription?
A persistent claim holds that the Ise mirror bears an inscription, variously reported as Hebrew or as a phrase resembling a divine name, and it circulates widely online. It originates in nineteenth- and twentieth-century speculation associated with the theory that the Japanese are descended from the lost tribes of Israel — a fringe idea with no linguistic, genetic or archaeological support, promoted by a small number of writers and never taken seriously by scholars in either country. Since nobody is permitted to view the object, the claim cannot be tested, which is precisely why it survives. Treat any specific description of the mirror's surface, from any source, as unverifiable, and note that the same has been true for every generation that has told these stories.