The Rance tidal power plant, situated between the cities of Dinard and Saint-Malo in northern Brittany, France, is a remarkable example of tidal energy technology. Completed in 1966, it generates about 500 gigawatt-hours of electricity annually—enough to power a city the size of Rennes. The plant features 24 turbines, each with a capacity of 10 megawatts, totaling 240 megawatts of power. These turbines are uniquely shaped like bulbs and equipped with adjustable blades that allow them to operate efficiently during both incoming and outgoing tides. On average, 80% of the plant’s electricity is produced during the falling tide. Each turbine must handle a water flow of 260 cubic meters per second—comparable to the flow of the Seine River in Paris. In addition to the turbines, the plant also includes six massive valves, each measuring 10 meters by 15 meters, that help regulate the movement of water. The Rance plant operates similarly to a dam, blocking the Rance estuary and controlling the flow of water. During high tide, water flows through the turbines to generate electricity before reaching the estuary. When the tide recedes, the water flows back through the same turbines, producing more electricity. This dual-direction operation makes the plant highly efficient. However, the cost of producing electricity at Rance is significantly lower than other sources, at about 0.018 euros per kilowatt-hour, compared to 0.0598 euros per kilowatt-hour for nuclear power. Despite its efficiency, the plant has had notable environmental consequences. By blocking the estuary, it has altered the natural tidal patterns, reducing the height of tides but maintaining a higher average water level. This change has disrupted local ecosystems, leading to the disappearance of species like lampreys and eels, while smaller, more adaptable species such as sea bass and flounder have thrived. The Rance tidal power plant is nearly unique in the world, given the specific conditions required for its construction. A suitable site needs a bay or estuary that can be closed, stable ground, appropriate depth, and strong tidal movements. The Rance River benefits from among the strongest tides in Europe, making it an ideal location. Globally, there are very few tidal power plants, and even fewer of this scale. The only comparable facility is the Sihwa Lake tidal power plant in South Korea, which began operations in 2011. In the United Kingdom, several tidal power projects have been proposed, such as the Swansea Bay project, but none have been completed. The Rance plant remains a pioneering example of tidal energy, highlighting both the potential and the environmental challenges of harnessing the ocean’s natural rhythms.