Kyozuka (経塚)
Kyozuka (literally, "scriptures tomb") are burial sites where Buddhist scriptures were buried. Burying scriptures was one of the sazen koi (good deeds in Buddhist terms), and the practice of building a kyozuka site was called maikyo, which was considered kuyo (a memorial service for the dead).
Summary
During the Nara period when Buddhist deities were believed to protect the nation, Buddhist scriptures were copied as a national project, but by the Heian period when Jodo-shiso (Pure Land Buddhism) prevailed, they were copied by individuals who wished for their prayers to be fulfilled. This practice is considered to be a form of belief of "mappo-shiso" (latter day pessimism, which prevailed in the middle of the Heian period, predicting that the mappo era ("latter day") would come in 1052), which may have originated in China or the Korean Peninsula.
Besides Hokke-kyo (Lotus Sutra), which were typically buried in kyozuka, "Hannya-kyo" (Heart Sutra), "Amida-kyo" (Sukhavati sutra), "Miroku-gyo" (Buddhist scriptures on Maitreya), "Dainichi-kyo" (Mahavairocana Sutra), "Kongocho-kyo" (Vajrasekhara Sutra), and "Rishu-kyo" (scriptures of esoteric Buddhism), were buried as well, and occasionally Kai-kyo (the beginning scriptures) and Ke-kkyo (the ending scriptures) like "Muryo gikyo" (literally, "sutra of immeasurable meanings") and "Kannon kenkyo" were also included. Scriptures were mostly copied onto paper, but occasionally onto clay tablets, copper plates, small stones, root tiles, or shells. Scriptures were usually contained in kyozutsu (cylinders made of metal) and inscriptions were carved onto them.
Kyozutsu were typically in the shape of small cylinders and sealed on the top, with variants being in the shape of hexagonal cylinders or decorative hoto (two-storied Buddhist tower). Some kyozutsu were contained in other containers made of metal, ceramic, or bamboo. Besides kyozutsu, Japanese mirrors, coins, blades, small Buddhist statues, precious stones were also buried, in addition to charcoal, which was to get rid of humidity. Burial sites were often located on the top of mountains or within the precincts of Shinto shrines, which were considered sacred, and a rock chamber was built under or on the ground and covered with soil. Some kyozuka were built in caves or steep-sided valleys, and by the middle ages, some were constructed by the wayside or in cemeteries to pray for the dead to rest in peace, which was considered to be a good deed.
The time when kyozuka came into existence up to the middle ages
The oldest kyozuka in Japan is the one built by FUJIWARA no Michinaga on the top of Mt. Kinpo in Yamato Province in 1007. Initially, kyozuka were built by nobles, who, out of mappo-shiso (latter day) pessimism, wished to prepare themselves for the advent of Miroku (Maitreya) and to hand down Buddhist scriptures to later generations.
After the 12th century, which saw its heyday, the practice of copying and burying Buddhist scriptures temporarily lost momentum until Kaikoku-hijiri (literally, "country-traveling saint") appeared in the middle ages, who traveled across the country and encouraged people to perform the practice. Thanks to these efforts, the practice prevailed among the common people, who prayed for the dead to rest in peace or wished their worldly wishes to be fulfilled.
Pre-modern ages
By the pre-modern ages, burying Buddhist scriptures by containing them in small kyozutsu was no longer popular. Instead, small, flat stones were used as popular media for carrying scriptures, and multiple stones with one letter from scriptures written on them, which were called "Ichiji isseki kyo" (literally, "one-letter stone scriptures"), or one stone with multiple letters from scriptures written on it, which was called a "Taji isseki kyo" (literally, "multiple-letter stone scriptures"), were buried. Even an isseki kyo (one-stone scripture) was buried in the ground and covered with soil, and a monument was elected for it to pray for the dead. In pre-modern ages, kyozuka were built as a part of religious activity, which had become popular as agricultural output increased and the money economy took root in society. The practice was led and guided by temples and many people participated who were motivated by the Tasu sazen shiso theory (a belief that good deeds will bring them more blessing).