What we don't get here is a sense of whether there really is an alternate secret postal system that serves a sort of underground of private networks.

在这里我们所不明白的是，是否真的存在一个,提供地下的,私密的通信服务的，秘密邮政系统。

耶鲁公开课 - 1945年后的美国小说课程节选

You know, there's been a lot of argument with the underground, for example.

比如，关于地铁的问题一直有很多争议。

伦敦的现代化 - SpeakingMax英语口语达人

After the passage is over, Milton addresses Alpheus, a river god -the god who was noted for plunging the course of his river underground in order to chase the nymph Arethuse who had been turned into a fountain.

这段结束后，弥尔顿引用了河神阿尔甫斯,那位因私底下掀起河流的波浪而闻名的神,以此来与被变成海底之泉的女神阿托瑞萨相互融合。

耶鲁公开课 - 弥尔顿课程节选