A loose series of novels and shorter works by Harry Turtledove, set in a world where the OTL eastern seaboard of the United States (including the Appalachias) broke off from America in prehistoric times and became a small island continent more to the centre of the Atlantic Ocean. Once European colonists arrive, this land is dubbed “Atlantis”.
The series has been criticised by the AH community for being occasionally a bit too parallelistic for its own good (a British colony created in Atlantis rebels in the 18th century and forms a vague United States analogue, there's a smaller-scale American Civil War analogue in the 19th century, etc.). However, it has also received a degree of praise for the better thought-out non-parallelistic aspects of its AH worldbuilding (the fauna and flora of Atlantis evolve in isolation and under evolutionary mechanisms similar to those of New Zealand, the geographic names and settlement of Atlantis is different to OTL and the colonising countries are often different too, including settlers descended from Basque fishermen, etc.).