n.底流；退浪；下层逆流
Large rocks line the base of the falls, and the man also avoided dangers like strong currents and a vicious undertow. 瀑布的底部是一块块巨石，该男子还逃过了强劲的水流和凶猛的回头浪。
The pull of the undertow can drag swimmer out to sea. 近岸回流的冲力能把游泳的人涌到海里。
The children were carried out to sea by the strong undertow. 强大的回流把孩子们卷到海里去了。
