How can a lagoon remain inside an atoll as the ocean floor sinks?

As the ocean floor subsides, a lagoon remains inside an atoll because the coral reef structure is able to maintain its upward growth at a rate that keeps pace with the sinking foundation.12

According to the theory proposed by Charles Darwin, atolls originate as fringing reefs around a volcanic island. As the island and the surrounding ocean floor subside, the reef grows upward and outward toward the open water. This process creates a barrier-reef stage, where the reef becomes separated from the volcanic shoreline by an intervening stretch of water.13

Once the volcanic peak is completely submerged, the reef continues to grow along its seaward edge, effectively enclosing a central area that becomes the lagoon. The lagoon maintains its depth through the accumulation of coral and calcareous sediment derived from the reef. While subsidence is a primary driver, fluctuations in sea level also play a significant role in the development and current appearance of these structures.1324