A mosaic representing the sea (Thalassa). From the Wikipedia: In Greek mythology, Thalassa (Θάλασσα "sea") was a primordial sea goddess, daughter of Aether and Hemera. With Pontus, she was the mother of the nine Telchines and Halia. Sometimes, she was thought of as the mother of Aphrodite with Uranus. She was the personification of the Mediterranean Sea.
Alternative names are Thalatta and Thalath.