n.底流；退浪；下層逆流

Large rocks line the base of the falls, and the man also avoided dangers like strong currents and a vicious undertow. 瀑布的底部是一塊塊巨石，該男子還逃過了強勁的水流和兇猛的回頭浪。

The pull of the undertow can drag swimmer out to sea. 近岸回流的沖力能把游泳的人涌到海里。

The children were carried out to sea by the strong undertow. 強大的回流把孩子們卷到海里去了。