Abstract: An illumination system for illuminating an underwater space is presented. The system comprises an elongated flexible electroluminescent light source located substantially inside the underwater space and a power supply unit located outside the underwater space. The power supply unit is connected to one end of the electroluminescent light source. At least this one end of the light source, which is connected to the power supply unit, is located outside the underwater space. The light source has at least one light-emitting element located inside an inner insulating sheath, and has an outer insulating sheaf. The inner insulating sheath and the outer insulating sheath are separated by a continuous air gap coupled to an air region outside the underwater space. This prevents the light source from being affected by water, and thereby from the reduction of its lifetime.