Weddellite (calcium oxalate dihydrate) is the main component of the alteration affecting every marble surface on the Isle of Torcello in the Venetian Lagoon. Its origin must be related to the presence of numerous cyanophites, whose role is to provide the system with oxalic acid, which in the presence of a carbonatic substratum precipitates in the form of weddellite. The enormous proliferation of these perforating algae, as of other endemic species, is connected to characteristics that are peculiar to the Venetian Lagoon. See also AATA 18-595. -- AATA