One of the islands in Venice's lagoon is Torcello, which was the first settlement in Venice's lagoon, before Venice existed. Now it's almost a ghost town, with just a handful of people living there among the fields and marshes. The main attraction is the very, very old Basilica of Santa Maria Assunta. Much of it dates from about 1008 (with some gorgeous mosaics from the 12th and 13th centuries), with bits from as far back as the 600's. It has a truly ancient feel to it.