The Marshall Islands look like somebody gave up building a racetrack in a game

Yeah, the Bikini Atoll was a testing ground for nukes, but that's not the reason they are strangely hollow outlines of islands.

As far as I understand it, it's because there were islands formed from underwater volcanic eruptions, but over time the inner islands sunk and eroded away and what makes up the island today are actually the surfaced parts of the once surrounding coral reef.