Shiranui

Shiranui (不知火^?, unknown fire, Shiranuhi in the historical kana orthography) is a atmospheric ghost light told about in Kyushu. They are said to appear on days of the noon moon such the kaijitsu (29th or 30th day) of the seventh month of the lunisolar calendary when the wind is weak, in the Yatsushiro Sea and the Ariake Sea. Furthermore, they can be seen in modern times, but they have been determined to be an atmospheric optical phenomena.

Several kilometers from the beach in the open sea, first, one or two called "oyabi" (親火) would appear. These would split off to the left and right and multiply, and in the end, several hundred to several thousand fires would be lined up in a crowd. It is said that they would even stretch to be 4-8 kilometers long. Also, the greatest number of shiranui would be able to be seen at the time of greatest ebb tide at around 3:00AM or within two hours of this time.

They are completely unable to be seen near the water, and it is said that they are able to be confirmed at a height of about 10 meters from the ground. Also, it is certainly impossible to get close to these shiranui, and by attempting to get close, these fires would go far off. They were once said to be the lamp of the dragon god, and nearby fishing villages would prohibit fishing on days when a shiranui was seen.

According to the Nihon Shoki, the Hizen no Kuni Fudoki, and the "Higo no Kuni Fudoki," when Emperor Keiko, went to southern Kyushu to conquer the natives and visited Kumamoto, it is said that they advanced using the shiranui as a landmark. In his journey for expanding the Yamato Ōken, saw unexplainable spots of moving fire, Shiranui, in the Ariake Sea and the Yatsushiro Sea (also called the Shiranui Sea), and a gozoku in that land replied that he did not know the fire, the origin of Shiranui. This light was named Shiranui ("unknown light") and the area was named Hi Province (火国 or 肥国^?, Hi no kuni, land or country of fire), now Kumamoto Prefecture.

...
Wikipedia