These are the undisputed trademarks of the city of Venice. They are oaken stakes planted in the seabed and used to reveal the waterways and tides to incoming vessels. On average the stakes in the lagoon last for 10 to 20 years, then corroded by the tides, they have to be replaced periodically. A main feature of the stakes, which also serve as a natural habitat for the sea's fauna and flora, consists in the traces left by shipworms. These are long shellfish which burrow through the wood, leaving perfectly round holes and go on to create designs and silhouettes which are very suggestive.