The Irazu Valley Mural is an Archive item found in Forbidden Siren. It is the first Archive item in numerical order. It is a mural that was discovered during the Meiji period in the formerly forbidden-to-enter Irazu Valley. It clearly shows Datatsushi on top of a triangular rock that appears to be the Mana Stone. It depicts three people seeing this and fearing and honouring him. It is estimated to have been drawn in the latter half of the seventh century, so obviously centres on the taboo event that took place in 684 AD, when the residents of Hanuda Village (three people including Hisako Yao) were starving during a famine and are said to have "consumed the flesh of a god". According to "Nihon Shoki #29": "June - ceremony to bring rain. July - Comet appears in the north-west. Roughly three metres long." However, what this literally means is, "(in the area of land where Hanuda Village is) Famine due to drought. Residents are in so much trouble that they perform the "rain-calling" rites, and a comet flies past." The comet can also be interpreted as "a mysterious flying object falling from space". In that case, it strongly hints that Datatsushi, revered as a god, is a "being from another planet".
The condition for obtaining it is to "learn everything", so once you have gathered 99 Archive items you will obtain it automatically.