Akaei

Sailors out of Awa — the southern tip of what's now Chiba — are caught in a storm, blown far off course, and left drifting with no idea where they are.

Then they see an island.

They land. It's a relief and it's also immediately strange. There are no people. The plants growing out of the rock aren't ones they recognize. There's seaweed hanging in the tops of the trees. There are fish living in the crevices of the rocks, well above where any tide should reach. They walk inland for something like ten kilometers looking for a village, a stream, anything, and find absolutely nothing, so they give up and go back to the ship.

And as they pull away from shore, the island sinks.

What they had been standing on

It was a stingray. A red stingray, in the telling, more than twelve kilometers across — so large that sand and silt settle on its back over time, until it surfaces to shed the load and, for as long as it's up there, is an island.

Ships that make landfall on it are destroyed when it goes down. And the picture-book that records all this, from 1841, adds the single most alarming sentence in the entry almost as an afterthought: fish like this are abundant throughout the ocean.

There's an even bigger relative on the books, described in a travel journal from 1797 — a sea-thing so vast it eats whales the way whales eat sardines, and of which no one has ever seen more than a fragment at a time. Japan, it turns out, has its own entry in a genre the whole seafaring world wrote independently: Sinbad moors on the whale, the Norse sailors meet the hafgufa, and the crew out of Awa walk ten kilometers across a fish.

Why this one is worse than a sea monster

Read those landfall details again, because they're doing something very deliberate.

The men check. That's the horror. They run the whole sailor's checklist for unfamiliar land — is there fresh water, are there people, does the vegetation make sense — and every single item comes back wrong. Unknown plants. No water. No inhabitants. Seaweed in the treetops, which can only mean this entire surface was recently underwater. The evidence is complete and it is in front of them for ten kilometers, and they do not act on it, because the conclusion is not available to a human being standing on solid ground.

Most sea monsters threaten the ship. The umibōzu rises out of the water and looms. The funayūrei come back for what the sea took. The akaei is doing something colder than either. It attacks the one thing a sailor never questions: that land is land.

The seaweed hanging in those trees might be the quietest terrible detail in all of Japanese folklore. Nobody in the story ever looks up and understands it.

The ordinary fish underneath

For what it's worth, the akaei is a real animal — the red stingray, a common enough catch in Japanese waters, entirely capable of being a normal size.

It carried an odd reputation even before the picture-books got to it. Its Edo-era nickname was keiseigyo, roughly "courtesan fish." The plain species was already strange in the popular imagination; the storytellers just scaled it up until you could put a forest on its back.