The ninja tradition centered on the neighboring areas of Iga and Koka is, Stephen Turnbull concludes, an invented tradition:
The ninja has become a familiar figure in popular Japanese culture as the world’s greatest exponent of secret warfare. He infiltrates castles, gathers vital intelligence, and wields a deadly knife in the dark. His easily recognizable image is that of a secret agent or assassin who dresses all in black, possesses almost magical martial powers, and is capable of extraordinary feats of daring. He sells his skills on a mercenary basis and when in action, his unique abilities include confusing his enemies by making mystical hand gestures or by sending sharp iron stars spinning towards them.
There is much popular support for the historical truth that is supposed to lie behind this familiar image. It is reminiscent of the passions displayed by the members of a religious cult, because like any cult the ninja’s loyal followers staunchly defend both his worth and his authenticity. Yet even the most devoted fans of what might be called “the ninja cult” will acknowledge that a certain amount of exaggeration has probably taken place. Similarly, only the most dogged ninja skeptic would dare to argue in an equally passionate manner that the idea is a total fabrication. The usual approach, even among scholars, is simply to accept the original ninja myth as a genuine historical phenomenon that has for centuries been greatly romanticized and, more recently, highly commercialized.
This modern exploitation of the ninja has proved highly profitable, eclipsing anything derived from Japan’s other great warrior tradition of the noble samurai, to whose example of loyalty the ninja provides a dark antithesis of secrecy and deception. The samurai have also been subject to exaggeration and commercialization in recent years, but whereas examples of the samurai tradition are to be found all over Japan, the modern ninja cult has one unusual feature in that its exploitation is concentrated in a very small area. This is the former province of Iga (now part of Mie Prefecture) and the place with which it shares a border, an area of modern Shiga Prefecture called Koka. The two places once had much in common and are often linked in the historical narratives. Nowadays Iga-Ueno City has by far the most developed ninja-related infrastructure, making it is the best place in Japan to visit a ninja house and a ninja museum, to enjoy martial arts displays, and purchase a wide range of ninja souvenirs.
[...]
All these points could apply to the development of the ninja, but if the ninja are indeed an invented martial tradition they are by no means alone, because military societies are particularly vulnerable to the creative approach. For example, the popular image of the pirate derives almost entirely from the American author and illustrator Howard Pyle (1853-1911). Pyle regarded the look of ordinary eighteenth century sailors as too dull for these romantic figures, so he added elements drawn from the Spanish folk costume of his day, especially headscarves knotted behind the head, large hooped earrings and wide trailing sashes, to a somewhat mistaken notion of authentic seafaring dress (Konstam, 2011, pp. 26-27). Nor is Japan exempt from such a process. The story of the vendetta carried out by the Forty-Seven Ronin of Ako derives almost entirely from an unashamedly fictionalized play that was staged shortly after the incident on which it was based (Turnbull, 2011, p. 5). Even the modern understanding of bushido, the virtually sacred “way of the warrior,” owes much to Bushido: The Soul of Japan, a book first published in 1900 by a Japanese Christian living in America (Nitobe, 1905). As bushido is such an important concept for defining the way of the noble samurai it is ironic indeed to consider that the cult of the underhand ninja may predate it by almost three centuries.
I must however declare an interest. It is not uncommon to regret the excesses of one’s youth, and to have produced a book where enthusiasm sometimes overwhelmed common sense is part of my own history. In Ninja: The True Story of Japan’s Secret Warrior Cult (Turnbull, 1991) I translated the historical sources that were then available and interpreted them as I understood them at the time. However, I allowed myself to become a little overdependent on compilations of these sources, in particular the books by Sasama (1968) and Yamaguchi (1969), neither of whom makes clear the age and reliability of the material they include. In their books contemporary descriptions of secret warfare are juxtaposed with accounts that were written well after the establishment of the ninja myth and therefore liable to have been influenced by it. In this article I shall therefore re-examine the evidence with a degree of academic rigor that may have been lacking in 1991.
For the purposes of this article, I propose that if the ninja have any basis in fact the following three criteria must be satisfied:
1. A unique corpus of military techniques involving secrecy existed in Japan during the
Sengoku Period.
2. The exercise of these techniques was confined to certain skilled individuals rather
than being spread more widely within Japanese society.
3. These skilled practitioners were identified in particular with Iga and Koka, from where
they sold their services to others.
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I therefore conclude that the authenticity of the Iga Koka ninja tradition fails against all three of the suggested criteria in terms of the existence of secret warfare, the elite practitioners of it, and its narrow geographical location. Undercover operations were performed throughout Japanese history but were carried out by skilled warriors who did not belong to a hereditary tradition. Iga and Koka did not have a monopoly, nor is there any evidence for a transmission from there to other provinces after 1581. In fact the opposite is true. If the presence of shinobi in an account indicates that secret warfare is taking place, then Iga and Koka are remarkable not for how many references there are to them but for how few. The ninja of Iga and Koka therefore present a classic example of an invented tradition in terms of Hobsbawm and Ranger’s definition. There is a deliberate “attempt to establish continuity with a suitable historic past” whereby ancient records have been re-interpreted and exaggerated to reinforce a highly localized understanding of a military phenomenon (Hobsbawn and Ranger 1983, p. 1). Hobsbawn and Ranger (1983) also identified the “use of ancient materials to construct invented traditions of a novel type for quite novel purposes” (p. 4). This seems to be what has happened at Shimabara.
I do not however believe that the Iga-Koka ninja myth or the modern cult that developed from it represent a total fabrication. All invented traditions have a basis in fact, no matter how tenuously the links may be made between the developed tradition and recorded history. In Iga and Koka there must have been some genuine belief in a unique local expertise that was bolstered by folk memories and old soldiers’ tales, and the best that can be said for their plagiarism of other people’s exploits is that it supports one great ninja stereotype: they were very good at stealing things! Yet even if the Iga-Koka ninja cult draws upon little more than the manipulation of folk memories and historical records, any tradition that takes shape in about 1620 and continues to the present day is worthy of more attention and respect as a cultural property than is commonly given to other aspects of the samurai tradition. As the Iga and Koka ninja tradition is older than the 47 Ronin and even predates bushido it should not be dismissed but celebrated as Japan’s oldest martial invention and, through its modern cult-like manifestation, as Japan’s greatest martial fantasy.
Musk’s Japanese steampunk-themed 42nd birthday party, Walter Isaacson explains (in his biography of Elon), culminated in a demonstration of Sumo wrestling: 
