Great Southern Pelagic Ocean
In the world of Nation, most of the geography is identical to our own, but often the names have changed. What we know as the South Pacific appears on their map as the Great Southern Pelagic Ocean. While the ocean in general is the same shape and disposition, the many chains of small islands are not in the same pattern as the Pacific's. One of these island groups is the Mothering Sunday Islands, including Nation. "Pelagic" refers to water not close to the bottom or shore, implying a large deep ocean, which it is.
Our Atlantic Ocean is not named on the Nation map, but of course they know we were all wrong about Atlantis.