Venice is built on a group of 118 small islands that are separated by expanses of open water and by canals, and portions of the city are linked by over 400 bridges. The islands are in the shallow Venetian Lagoon, an enclosed bay lying between the mouths of the Po and the Piave rivers. When the new settlers arrived on the islands around 402 A.D., they had to find ways to strengthen the islands, drain them, enlarge them, and protect the fragile environment. They dug hundreds of canals and shored up the banks with wood pilings, which were put as close together as the soil would let them, and the spaces between them were filled with rocks and stones to keep the silt from rising during the next piling. On top of the piles, two layers of wood were added, and this was the start of the masonry work, which often used limestone because it wouldn’t let water through. All buildings in Venice were built using four materials: wooden piles, limestone, brick, and Istrian stone. The wooden piles were hammered about 4 meters deep to solidify the soft ground. The limestone layer was placed above these piles, and this served as a solid base for the structure above and protected the wooden piles from the erosive action of the seawater. Today, Venice faces problems like rising sea levels and erosion caused by boats in the canals, but the city is working hard to ensure it stays for years to come.