Ishiko no Dōkutsu

The Cave of the Stone Children was a coastal, rocky and treacherous cave near Akagi Mori in the Umoeru Province. The water in the cove had strange currents that both allow detritus to wash up onto the sand and can drag a person underwater, deep into sharp-edged tunnels. The cave extended deep into the cliff face, the air becoming strangely ashy and thick like fog. The tunnels, some flooded, were small, claustrophobic spaces, full of stalagmites and stalactites, jagged stones, and random twists that disoriented visitors.

Tsunami
In 1120 a shrine used for good-luck offerings and childrens' blessings was destroyed by a tsunami. Days later, the shrine's name scrolls and placards washed up in the cove, along with small stones with pits and markings that resembled a sleeping baby's face. A couple, Asaji and Taketoki, heard about the stones. Believing they were reincarnated children, they went there hoping to find the stone that was their own lost child. They took all the stones, scrolls, and placards and placed them in a cave entrance within the cove. In their grief, however, they believed that all of these "stone children" were theirs. They prowled the area ambushing travelers, thinking them there to kidnap their family. In addition, the two had taken to stealing food, offerings, and clothing from nearby villages, and a few villagers had gone missing. Those villagers were all ones who had lost their own children, and most of their peers simply believed they had flung themselves into the ocean. The couple had been planning on kidnapping real children as a sacrifice or to raise as their "own" children.