Correct

地下 (dì xià) vs 地下 (dì xia)

/ dìxià vs dìxia /
Strokes

Definition

地下 (dì xià) is a Noun, which means “underground”;

地下 (dì xia) is the noun plus a noun of locality, meaning “on the ground”.

Example

地下 (dì xià), a noun, means "underground"; figuratively it means "some secret, hidden activity".
The usage in example ① is the original meaning, and that of ② and ③ are figurative meaning.
1
Zhège
这个
jiànzhù
建筑
shang
地上
céng
  
,
xià
地下
liǎng
céng
  

There are eight stories above the ground and two under the ground of the building.

2
Zhànzhēng
战争
shí
时期
zēng
cóngshì
从事
guò
xià
地下
gōngzuò
工作
  

He once worked as a spy in the wartime.

3
Àn
案发
hòu
  
,
zhuǎn
转入
xià
地下
  

After the accident, her activities became secret.

地下 (dì xia) is the noun 地 plus a noun of locality, meaning "on the ground (also 地上 (dì shang) meaning, having the same).
4
Yǎnjìng
眼镜
diào
zài
xia
地下
  
(
shang
地上
  
)
le
  

The glasses fell on the floor.

5
shang
地上
  
(
xia
地下
  
)
shì
shénme
什么
dōng
东西
  
?
Bàn
le
xià
  

What's on the floor? I stumbled over it.