Yomi is the world of the dead. The term originally meant 'underground spring' in classical Chinese, but its meaning gradually changed to mean the underground world of the dead. In Japanese, since according to Japanese myth the name of the place where the land of the underground spring existed was called "yomi" (lit. "night-see"), it has been proposed that the word "yomi" is actually a corruption of yume (dream). So when the word "yomi" arrived in Japan, it became linked with the land of the dead as conceived by the Japanese at the time. Another theory holds that yomi comes from "yomo" (all four directions/sides), simply expressing the concept that it is what lies beyond the world of everyday life. Aside from this, there is a separate view that Yomi originated from Yami (闇). Another theory holds that the Koyomi (almanac/calendar) which derives from Tsukuyomi (moon-reading), originally a method to calculate the number of days since the new moon, was used to count the number of days until the new year--when the spirits of one's ancestors return from Yomi as Toshigami (gods that dwell in one's house during the new year holiday).
Myth or theories concerning Yomi
It was believed that ancient Japan had a path to Yomi and was connected to Ashihara Naka no Kuni (fictional Japan) by Yomotsuhirasaka (Slope of Yomotsuhira). Izanagi pursued his dead wife Izanami down this path and entered Ne no Katasukuni (Kata Province), which is generally though to be the same as Ne no Kuni (the underworld, lit. "Root-Land"; today, Kata Province is the area mainly occupied by the city of Yasugi in Shimane Pref. This is also said to be the origin of the word "Shimane" (Island-Root). Yomotsuhirasaka is presumed to be equivalent to the town of Higashiizumo, which is next to Yasugi).
But he broke his promise to his wife, and had to watch as she of such beauty was transformed into a corpse devoured by maggots; seeing this, Izanagi became afraid of her and returned to the surface, escaping from Yomi in the manner described below.
Izanami chased the fleeing Izanagi as far as the sakamoto (foot of the hill) of Yomotsuhirasaka (some manuscripts record a different alternate version of this passage). On she chased after his fleeing form, until they reached the foot of Yomotsuhirasaka.
In a more colloquial translation, "Izanami kept chasing Izanami until (he) reached the foot of Yomotsuhirasaka," where "foot" (sakamoto) refers to the foot of both a downhill and an uphill path. Consequently, it is possible to interpret this passage as claiming that Izanagi ran down (not up) the Yomotsuhirasaka. Therefore, the country of Yomi was not always located underground in comparison to Ashihara Naka no Kuni.
In order to avoid both Izanami, still hot on his heels, as well as the shikome (ugly women) underlings of Yomi, he placed a big boulder to block the path to Yomi, and this boulder was called "Chigaeshi no Okami" (the great god of Chigaeshi); the half of the path to Yomi that remained connected to the world of the living is thought to be located in Ifuyasaka (the present-day city of Higashiizumo in Shimane Pref.).
Moreover, he drove off the shikome of Yomi by pelting them with peaches he took from a tree growing on that spot, so he gave the peach the divine name "Ookamusubi no mikoto", saying "May you ever protect those in need (just as you have done for me now)."
Ne no kuni was also believed to be located in Kumano due to the theory that the land of Yomi was actually Kumano Sanzan, but comparing the accounts given in the Kojiki and the Izumo fudoki (Regional Gazeteer of Izumo), the belief that Yomi is located in Izumo is more plausible.
More than a few academia believe that Yomi Country is located underground and this is accepted by the present day public. However, the definition of the land of the dead being underground came from the Chinese Yomi and did not follow the original Japanese thought. Yomi referred simply to the location of Ne no kuni, and as such the theory that it was located somewhere between the town of Yomi in the city of Yonago, Tottori Prefecture, and the town of Higashi Izumo (where Yomotsuhirasaka was located) in Yatsuka county of Shimane Prefecture is most plausible; excavations nearby have unearthed huge square tombs built in a very early period of Japanese history (for example, Tsukuriyama tomb in the city of Yasugi in Tottori) as well as sokantotachi (swords) including an iron sword that closely resembles Kusanagi, one of the three Imperial Regalia.
The Chigaeshi no Ookami is worshipped as the god that protects travelers everywhere around Japan.
But the biggest reason that Izumo province was thought to be the location of Yomi, the underworld, is that it was believed to be the place where the sun set on the Japanese archipelago.
This matched the folklore passed down in Kiki concerning the southern sky of the Kinai (in and around the capital) Region.
Nihonshoki (The record of Japanese history commissioned by the emperor)
The sixth passage of the first volume of the "Nihonshoki" reads, "A certain place is called "Izumitsuhirasaka," nor again is this separate from the existing places, but when the breath of people stops in death, is this its name?"
Izumo no Kuni Fudo Ki (The Endemic Record of Izumo Prefecture)
The paragraph on the Uka area of Izumo district in the "Izumo no Kuni Fudoki" (Regional Gazeteer of Izumo) records that there was a cave called Yomi no Saka (Yomi Slope) and/or Yomi no Ana (Yomi Cave); the entry reads as follows: "People cannot reach it, and do not know (its) true depth; and all those who reach this underground grotto in their dreams die without fail." It is an accepted theory that this cave was 'Inome Dokutsu' (Inome Cave) located at Inome-cho, Izumo City. Inome Dokutsu was excavated in 1948 and human bodies and burial goods that ranged from Yayoi period to Tumulus period were discovered.
Yomi in China
Commonly, Chinese people called Yomi as Kosen (Chinese: Huáng quán) in Chinese reading style.
The ancient Chinese believed that an underground world of the dead existed; they called this world Huangquan ("yellow spring," pronounced "Yomi" or "Kosen" in Japanese). The Japanese kanji for "yo" (黄) in "Yomi" (黄泉) symbolizes earth in the concept of the five elements and was used when describing things that were underground.
Yomi within the Bible
When the Bible was translated into Chinese, the Greek word Hades in the New Testament and the Hebrew word "Sheol" in the Old Testament were both translated Huangquan (yellow spring/Yomi), and the Japanese translation also rendered these words as either Yomi or--using the same characters--pronounced it Kosen, following the Sino-Japanese reading of the term. Such places were thought to be a sort of limbo, realms between heaven (and hell) and earth where the dead go to await the Last Judgment. The following is Yomi as portrayed in the Bible.