Sentence Completion

Complete the sentence with ONE word from the passage for each blank.

Venice has long struggled with flooding caused by exceptionally high tides, locally called acqua alta. To protect the historic city, engineers built a system of mobile barriers at the mouths of the lagoon. When a dangerous tide is forecast, the hollow gates fill with air and rise from the seabed, holding back the rising sea. Once the threat passes, the gates are filled with water again and sink quietly back to the bottom, reopening the lagoon to shipping.

Venice's flooding is driven by unusually high . The protective gates rise when filled with , and they sink once filled again with .