The inhabitants of Marseille were well aware of the following situation:
During heavy stormwater events, thousands of cubic metres of waste water and storm water were discharged into the sea due to the lack of rain retention basins.
The Calanque de Cortiou was then closed for the public. The ecological challenge was to prevent the release of one million cubic metres of waste water per year into the natural environment and to relieve the Geolide wastewater treatment plant, the largest underground plant in Europe (40,000 m²), by the construction of a huge wet weather flow retention basin.
