- A method of desalinating sea water, comprising: - providing a sealed reservoir configured to be located at the seaside, comprising a filling device on the roof and one or more communication openings configured to be located below sea level and communicating with the sea; - filling the entire reservoir with seawater through said filling device, said communication openings being sealed, followed by closing said filling device to seal the reservoir; - progressively opening said communication openings to communicate the water in the reservoir with the sea water, the level of the water in the reservoir dropping and stabilising at a height h above sea level, the height h being defined by (Pa-Ps)/(pg), Pa representing atmospheric pressure, Ps representing the pressure due to the saturating vapour at the temperature of the sea water in the reservoir, ρ representing the mass density of water, g is the acceleration of gravity, water vapour being produced in the reservoir above the height h and the reservoir has a headroom above sea level equal to the height h plus the additional height which gives the space required to handle the water vapour produced in the reservoir; - condensing the water vapour produced, to obtain desalinated water; and - maintaining a so-called "flash" evaporation phenomenon in the reservoir by condensation of the water vapour produced and evacuation of the water from the surface of the reservoir to the sea, the height h of the water in the reservoir being maintained by the communication openings which bring the sea water inside the reservoir into contact with the water of the sea after the filling phase. - providing a sealed reservoir configured to be located at the seaside, comprising a filling device on the roof and one or more communication openings configured to be located below sea level and communicating with the sea; - filling the entire reservoir with seawater through said filling device, said communication openings being sealed, followed by closing said filling device to seal the reservoir; - progressively opening said communication openings to communicate the water in the reservoir with the sea water, the level of the water in the reservoir dropping and stabilising at a height h above sea level, the height h being defined by (Pa-Ps)/(pg), Pa representing atmospheric pressure, Ps representing the pressure due to the saturating vapour at the temperature of the sea water in the reservoir, ρ representing the mass density of water, g is the acceleration of gravity, water vapour being produced in the reservoir above the height h and the reservoir has a headroom above sea level equal to the height h plus the additional height which gives the space required to handle the water vapour produced in the reservoir; - condensing the water vapour produced, to obtain desalinated water; and - maintaining a so-called "flash" evaporation phenomenon in the reservoir by condensation of the water vapour produced and evacuation of the water from the surface of the reservoir to the sea, the height h of the water in the reservoir being maintained by the communication openings which bring the sea water inside the reservoir into contact with the water of the sea after the filling phase.