Underground
Basement, Cellar
noun, の adjective
Below the ground is underground.
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.
その車両は地下に入った。
The vehicle went underground.
地下への入り口はどこですか？
Where is the entrance to the basement?
デパートの地下のことを、「デパ地下」と言います。
The basement floors of department stores are called "depachika".