In 1994, EDF, a major French energy company, began operating the Petit-Saut dam with the goal of providing renewable energy to an isolated region, avoiding the pollution typically associated with thermal power plants. However, the construction of the dam led to the submersion of between 300 and 365 square kilometers of primary tropical forest, an area known for its incredibly rich biodiversity. The flooding created an unusual environmental phenomenon that continues to intrigue scientists today. As the submerged vegetation decomposed, it led to a lack of oxygen in the water, the death of fish, and significant emissions of methane, a potent greenhouse gas that is 25 times more effective at trapping heat than carbon dioxide. The scale of deforestation at Petit-Saut is staggering when considering the density of life in equatorial forests. A single hectare can contain an enormous number of species, from trees to insects. At the time of the dam's construction, the decision to clear the entire area before flooding was not fully considered, resulting in unforeseen environmental consequences. Scientists began monitoring the lake soon after its creation, making Petit-Saut one of the first tropical reservoirs to be studied in such detail. The reservoir holds a vast amount of carbon, with an estimated 3,690 gigagrams in the soil, 4,950 gigagrams in tree trunks, and 180 gigagrams in leaves. This organic material, once part of a thriving forest, now decomposes in a completely different environment, submerged in water and deprived of oxygen. The process has been likened to a massive compost heap, where the trees, leaves, and soil continue to rot and ferment in the darkness of the lake. The bacteria in the water consume oxygen to break down the submerged vegetation, leading to anoxia—conditions with little to no oxygen—in the deeper parts of the reservoir. The decomposition also releases hydrogen sulfide, carbon dioxide, and methane, which caused the death of many fish shortly after the dam was filled. The combination of high temperatures, sulfate-reducing bacteria, and the lack of oxygen created ideal conditions for methane production. This gas then traveled downstream, deoxygenating the Sinnamary river for nearly 40 kilometers, affecting aquatic life far beyond the lake itself. To prevent ecological damage downstream, EDF installed an artificial aeration system, a kind of cascade designed to reintroduce oxygen into the river water. Research showed that methane emissions occurred through two main mechanisms: ebullition, where bubbles of methane rise from the lake bed, and diffusion, a slower exchange of methane between the water and air. A third, less visible process also contributes—sudden releases of dissolved methane when water passes through turbines or flood gates. Over the ten years following the dam's filling, the carbon stored in the soil and forest floor became the main source of these emissions. Globally, hydropower reservoirs are estimated to emit about 51 teragrams of carbon annually, with around 13.3 teragrams in the form of methane. Petit-Saut is not an isolated case but an example of a broader phenomenon affecting large dams in tropical regions. More than three decades after the dam was filled, Petit-Saut remains a key site for studying the long-term evolution of tropical reservoirs. Methane emissions peaked in the early years but have since declined as the available organic matter has been depleted. While this long-term decrease is reassuring, it underscores an important reality: the environmental impact of a tropical hydroelectric dam cannot be assessed solely based on its stable operating phase. The first few decades, marked by high emissions, must also be considered. Ongoing scientific research at Petit-Saut includes monitoring stations both upstream and downstream of the dam. These studies provide crucial data that help scientists refine their models and highlight a key point: when considering new dams in tropical forests, the issue of deforestation before submersion must be addressed with the utmost seriousness, given the long-lasting and significant climatic consequences of submerging a living forest. The history of Petit-Saut illustrates the complexity of distinguishing between renewable energy and truly clean energy. Thirty years after its commissioning, this dam in French Guiana continues to offer valuable lessons to the global scientific community about the hidden environmental challenges of tropical hydropower. As many countries still consider building large dams in equatorial forests, the question arises: can we still call such energy sources clean if we ignore the environmental impact of the submerged forest?