You won't find the Sunrise Islands on any of our maps. It exists in a parallel universe in the middle of the Great Southern Pelagic Ocean. You won't find that on a map either, but if you did, it would be somewhere between the United States and England.
Wherever it is, Mau calls it home. But by the end of the novel, he realizes that it's not just important to him: it's important to the world: "The Nation had been old, older than the [coral] reef" (12.58), over "a hundred thousand years old" (15.78). And it might just be the cradle of civilization, the birthplace of humanity.
The island and Mau are closely connected, and not just because Mau is the only resident still living. Early on, we see the island "Had been stripped of all its trees but one, which was a ragged stem with, against all hope, a few leaves still on it" (2.3). Hey, guess who else is the only one of his/its kind left? Yep, that ragged stem might as well be Mau, standing against all odds.
Mau's journeys across the island mirror his inner journeys through different roles in life. He starts on the Boys' Island, but soon learns the ways of the Men's Place, also known as the Grandfather's Cave. He even tentatively explores the Women's Place, which was "like the moon; [Mau] knew where it was but didn't even think about going there" (3.24). Just wait about a century, Mau!
As more people arrive on the island, things start to change. It's no longer just Mau and Daphne, but a whole—well, nation. The island becomes a kind of floating village—but one of children without parents, parents without children, wives without husbands, people without all those things around them that told them what they were" (7.11).
All those words—children, parents, wives, husbands—are relational words, and what are you when you don't have those things to compare yourself to? According to Nation, you're part of a community. You're the Nation: "People would live and die but there would always be a Nation" (4.68).