is a paranormal phenomenon or yōkai from Japanese folklore. Other names include or . Little is known of the origin of umibōzu but it is a sea-spirit and as such has multiple sightings throughout Japan. Normally, umibōzu appears to sailors on calm seas which quickly turn tumultuous. It either breaks the ship on emergence or demands a bucket or barrel from the sailors and proceeds to drown them. The only safe way to escape an umibōzu is to give it a bottomless barrel and sail away while it is confused.
Concept
They appear and disappear in the oceans, often at night, and it is thought that they would suddenly appear on what was previously a calm sea surface as a giant's black bōzu head and destroy ships. They are often a few meters (yards) to a few tens of meters (yards) in length, so they are seen as quite large, but there are also tales about relatively small ones.
Like funayūrei, there are often many tales that seem to be about hallucinations, and often the difference between them is not clear. They would say "lend me a ladle," and sometimes the funayūrei that come to sink ships are considered the same as umibōzu. However, unlike the funayūrei who come with stormy weather, umibōzu sometimes appear at seas with no abnormalities (and in this case, once the umibōzu was seen, the weather would start getting stormy), so it's pointed out that these could be things that do exist but were misinterpreted. Some examples of things that could have been misinterpreted include sea organisms, cumulonimbus clouds, big waves, and other natural phenomena.
Also, umibōzu would often come in swarms as naked bōzu-like beings and attack ships, and they would do things such as cling on to the hull and scull as well as put out the basket fire. It is said that they would sometimes let out a voice saying "yaa yaa" while swimming, and when hit with the scull would let out shrieks of "aitata". Their weakness is tobacco smoke, and it is said that if one is unlucky enough to encounter one, having one of those ready can lead to being saved.
Legends by area
In the Tōhoku region, there is a custom of sacrificing to the deities the first fish caught when fishing, and it is said that if this is not followed, an umibōzu would appear and destroy the boat and kidnap the boat owner.
The nurarihyon that appears often in Bisan Seto has a large round head and as they float toward the boat, they would then slowly sludge (nurari) away and then unexpectedly (hyon) float approaching the boat again. They would do this several times over to torment people.
In Cape Shiriya, Higashidōri, Shimokita District, Aomori Prefecture, it is said that people eaten by sharks would become "mōjabune". They can be repelled by melting some miso and letting it into the sea. The "umikozō" told about in the Kamo District, Shizuoka Prefecture is a boy covered with hair all the way to the sides of the eye, and it is said that they would approach fishing lines with a grin. Also, in Mikonohama, Kii Province, a small animal called the "mokuri kokuri" that resembles a weasel would go to the mountains on March 3 and to the sea on May 5, and they have the shape of a human, but would expand and contract at will and disappear as soon as they appear, and extract from the butts of people who come to wheat fields at night. They have a jellyfish-like shape, and are said to come floating in swarms above the ocean. During the Mongol invasions, they were considered to be the spirits of those who died in water, and their name was ateji for Mongolian Koguryo. In the Kitauwa District, Ehime Prefecture, the sea would become white at night and a "shirami", also called "shirami yūren", would come swimming, and fishers would call these idiots. However, it is said that if they hear "idiot", they'd get angry and cling on to the scull and give a bad time. On Sado Island, the "tateboshi" are a monster said to stand at a height of who would aim at ships and try to flip them over.
Umibōzu are also said to change their appearance, and in Kesennuma Ōshima, Miyagi Prefecture, there are tales of them shapeshifting into a beautiful woman and engaging in swimming contests with humans. There is also a similar tale in Iwate, but there it is said that those who accept the challenge would be swallowed down instantly. In Uwajima, Ehime Prefecture, there are tales where they would shapeshift into a zatō (blind person) and kill human women. Also, while there are many legends of them attacking humans, in Uwajima there is the legend that those who see an umibōzu would live a long life.