Abstract: A reverse osmotic desalinisation system including a desalinisation unit having at least one semi-permeable membrane in direct contact with a body of saline water at a depth d1, an interior chamber in fluid connection with the saline water via a semi-permeable membrane, the interior chamber being in fluid connection with an atmosphere above a surface of the saline water wherein the depth d1 provides a sufficient hydrostatic pressure p1, being greater than the osmotic pressure to effect at least some desalinisation of the saline water through the semi-permeable membrane to provide a permeate being substantially desalinised water into the interior chamber and a solute outside the interior chamber, such that a solute is able to passively dissipate into the surrounding body of saline water.