Urashima Tarō
The concept has origins in ancient Japanese literature, particularly the story of Urashima Tarō. It is about the fisherman Urashima Tarō, who saves a turtle and is brought to a wondrous undersea kingdom, but the story has a twist: after spending what he believed to be four to five days there, he returns to his home village only to find himself 300 years in the future. Other precursors to isekai include portal fantasy stories from English literature, notably the novels "Alice's Adventures in Wonderland" (1865), "The Wonderful Wizard of Oz" (1900), "Peter Pan" (1902), and "The Chronicles of Narnia" (1950).