Despite their comparatively small area, globally, mangroves have a disproportionate importance in providing environmental services in the tropics. Recent decades have seen dramatic advances in our ability to quantify the services of coastal lagoons, including (a) coastal protection and risk reduction from extreme weather, (b) fisheries, (c) habitat for migratory waterfowl and birds in general, and (d) mitigation of the effects of sea-level rise. Thanks to these studies, significant progress has been made in advancing conservation efforts in coastal lagoons despite strong opposition from coastal developers. More recently, a number of studies have highlighted the importance of mangroves in sequestering and storing carbon within their sediments. While some mangrove forests accumulate carbon as amorphous organic matter mixed with clay, others form deep layers of peat (partially decomposed but recognizable root tissues). Peat-forming mangroves have been accumulating root remains for thousands of years and harbor belowground carbon contents that can reach many thousand tons in one hectare. Their depth–age curves are invaluable records of sea-level rise during the Holocene. Retention of belowground peat allows some mangroves to accrete vertically and keep pace with sea level rise by growing on their own root remains, and in doing so they have been storing large amounts of carbon in their sediments. The quantification of total belowground carbon allows us to assess the importance of mangroves as carbon sinks, but, perhaps more importantly, radioisotope dating of peat fragments allows us to estimate the long-term rate of carbon sequestration in coastal lagoons.