Guided boat grotto on the south side of the Riou archipelago near Marseille, where a lit cave opens onto a deeper dropoff before the Impériaux rocks. It is a classic Calanques training and photo dive with Mediterranean fish life, gorgones, and a route that stays rewarding for beginners while still giving experienced divers deeper relief. Use a local Marseille boat operator and pick a calm-weather window, because the site can feel very different when the wind or swell comes up.