an area of low land that was once under the sea but that has been separated from it by dykes (= walls or channels built to prevent water covering an area)

In the Netherlands, millions of people live in drained polders below sea level. 在荷兰，数百万人生活在海平面以下、经排水形成的圩区。

More than 2500 km of dykes have been constructed to protect the Dutch polders from flooding. 已修建了2 500多公里长的堤坝来保护荷兰的圩区不受洪水侵袭。