CANADA: Several unassuming rock walls along the shore of Quadra Island are actually the ruins of ancient clam gardens constructed by First Nations peoples thousands of years ago. The walls were erected within intertidal zones to create sandy terraces, ideal habitats for shellfish such as littleneck and butter clams. Radiocarbon dating of organic material sampled from one wall indicates it was built nearly 3,500 years ago, making it the oldest known aquaculture system of its kind.