Established between the 5th and 6th centuries, the island of Torcello boasts the olderst building in the lagoon-the cathedral of Santa Maria dell'Assunta. Founded in AD 639, it contains some splendid ancient mosaics. The adjoining church of Santa Fosca, of pure Byzantine design, is another mark of Torcello's former glory-before being eclipsed by Venice it had a population of 20,000.