Research published in *Science Advances* indicates that a significant expansion of coral reefs occurred between 20 and 10 million years ago, primarily between Australia and Southeast Asia. This period marked the largest growth of coral reefs in the past 100 million years, leading to the establishment of the Great Indo-Australian Miocene Reef System. The study reveals that environmental, biological, and tectonic changes facilitated this expansion, creating new habitats that allowed for the evolution and diversification of marine species. The region, now known as the Indo-Australian Archipelago, is recognized as a global biodiversity hotspot, particularly the Coral Triangle. Fossils, geological records, and genetic data were utilized to uncover these findings, highlighting the crucial role of reef expansion in generating marine biodiversity. Interestingly, the largest modern reefs are not located in the same area where this ancient expansion occurred. Instead, remnants of this extensive reef system can be found off the northwestern coast of Australia, suggesting that the roots of today's marine biodiversity may lie in these ancient ecosystems. The findings emphasize the importance of understanding historical ecological changes to comprehend current biodiversity.