Serious underground storage (5)
I believe the answer is:
grave
This could be a double definition.
'serious' is the first definition.
(both can mean solemn)
'underground storage' is the second definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I don't see?
Can you help me to learn more?
(Other definitions for grave that I've seen before include "French accent; very serious" , "Dignified" , "Solemn; tomb" , "Grim-faced" , "foreign accent" .)