前面用an，例如：
1、There's an underground passageway that connects the five buildings.有一条地下通道连接着那5栋大楼。
2、They are accused of organising and financing an underground youth movement.他们被指控组织并资助地下青年运动。
3、There was an Underground map above one of the windows and I counted the stops to West Hampstead其中一扇窗户上方有一张地铁线路图，我数了数到西汉普斯特德站共有几站。
