His idea started when he heard two religious workers playing music for free in New York City's underground transportation system.

VOA: special.2009.07.31

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

On Monday,two suicide bombings on Moscow's underground train system killed thirty-nine people.

VOA: special.2010.04.03

"These difficult to reach populations frequently go underground and remain not visible to the medical delivery system,"

VOA: standard.2009.11.15

looked at things like where should they locate waste water treatment facilities because of the impacts of climate change, what risks are likely to occur to their subway system because it's underground and already requires enormous pumping of water during certain large rainfall events."

VOA: standard.2010.06.14