Subterranean
/səbtəˈreɪniən/
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Definition
(adj) Below ground, under the earth, underground.
Sentence Examples
Yanni visited a subterranean tunnel.
For thousands of years, the Martians had to live enclosed in buildings or subterranean structures.
CEFR Practice Quiz
We explored the ____ caves that stretched for many miles beneath the surface of the mountain range.