 As the island subsided, a coral reef around the former volcanic island was able to maintain itself near sea level by growing upwards.That reef is now over thick (in the lagoon, , comprised mostly post-Miocene limestones with a layer of upper Miocene (Tertiary "g") sediments and lower Miocene (Tertiary "e") limestones at the bottom overlying the basalts).What remains today is a shallow water atoll about across.