In 1966, an alumina plant near Marseille was granted permission to discharge its industrial waste directly into the Mediterranean Sea. Today, more than fifty years later, the Cassidaigne canyon, located just a few kilometers from the famous Cassis calanques, still holds about 32 million tons of red sludge, a byproduct of alumina production. This waste, rich in metals like titanium, chromium, vanadium, iron, and lead, has created a barren, lifeless zone in the canyon, with no aquatic life present over a large area. While solid discharges into the sea were banned in 2016, the legal battle over the environmental damage continues, with local groups, fishermen, and residents accusing the practice of being an act of ecocide. The alumina plant, established at the end of the 19th century, produces alumina by chemically processing bauxite, a type of ore. This process generates red sludge, a highly toxic residue. As onshore storage areas became full, studies were conducted by organizations like the ISTPM and local universities to assess the feasibility of deep-sea disposal. A pipeline was then constructed to transport the sludge to the Cassidaigne canyon, 7.7 kilometers from the coast, where it was released at a depth of 320 meters. This method allowed the plant to continue operations without disrupting local communities. Estimates of the total amount of sludge discharged vary, with some sources suggesting over 20 million tons between 1967 and 2015, while others cite a total of 32 million tons. The red sludge contains high concentrations of toxic metals, including titanium, chromium, vanadium, iron, and lead, and is also deficient in manganese, a nutrient essential for marine life. Environmental groups have long criticized these discharges, highlighting their long-term impact on the marine ecosystem and the accumulation of harmful substances in the ocean. In 2015, the case took a political turn when the regional prefect authorized the company Alteo to continue discharging sludge for an additional six years. This decision came at the same time as the COP21 climate summit in Paris, a global event focused on environmental protection. Over the past fifty years, the consequences of this practice are now visible in the Cassidaigne canyon, where the absence of marine life contrasts sharply with the expected biodiversity of the Mediterranean. Since 2016, discharging solid sludge into the sea has been prohibited, and the plant has returned to storing waste on land in the Mange-Garri valley. However, Alteo, which was granted a special permit until 2021, exceeded its storage limits much earlier, in 2019, before entering judicial reorganization and eventually ceasing operations. The legal case has now reached the courts, with environmental groups, local fishermen, and residents united in their belief that the buried waste in the Cassidaigne canyon should not be forgotten as a mere footnote in Provençal industrial history.