Sea Fall was a planet in the Demilitarized Zone between the Cardassian Union and the United Federation of Planets. It was the site of a Federation colony, ceded to the Union by treaty in 2370, though rather than accept resettlement, the colonists gave up their Federation citizenship to stay.
In 2373, Sea Fall was one of the many worlds in the DMZ attacked by Cardassian and Jem'Hadar forces in the Maquis purge just before the outbreak of the Dominion War. (DS9 novel: The Never-Ending Sacrifice)