Vegetated environments from forests to peatlands store carbon in the soil, which mitigates climate change. But which environment does this best? In this study, we show how the levees of tidal marshes are one of the most effective carbon sequestering environments in the world. This is because soil water-logging and high salinity inhibits carbon degradation while the levee fosters fast vegetation growth, complimented also by the preferential settlement of carbon-rich sediments on the marsh levee.