Just a few miles from Bora Bora among the Leeward Islands of French Polynesia lie Taha’a and Ra'iātea. The lush, inhabited tropical islands in the middle are remnants of eroded volcanos, while the keyhole-shaped network of turquois fringe encircling them are separate kingdoms altogether –living, evolving coral reef.
Communities of coral animals thrive in the warm, shallow waters around tropical volcanic islands. Even as the elements erode away the exposed volcanic land mass above the water, countless generations of coral go about their business building vast underwater cities of astounding biodiversity beneath the surface. Islands in tropical waters may erode away entirely, leaving behind only the surrounding reef – an atoll - to mark its former location.
Taha’a and Ra'iātea probably comprised a single, larger island before millennia of erosion split them apart.
Science Link: https://www.coris.noaa.gov/about/what_are