Jeju was already divergent from Seoul Korean by the fifteenth century, and was unintelligible to mainland Korean visitors by the sixteenth century. The language was severely undermined by the Jeju uprising of 1948, the Korean War, and the modernization of South Korea. All fluent speakers remaining in Jeju Island are now over seventy years old. Most people in Jeju Island now speak a variety of Korean with a Jeju substratum. The language may be somewhat more vigorous in a diaspora community in Osaka, Japan, but even there, younger members of the community speak Japanese. Since 2010, UNESCO has designated the language as critically endangered, the highest level of language endangerment possible. Revitalization efforts are ongoing.