I’m off to the underground shopping mall.
An underground (地下) street isn't as underground streety as it sounds. A lot of the underground streets and areas in Japan tend to be underground shopping malls, though. That or subways/trains, but underground shopping centers are way more interesting.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.