1 Old HSK word(s): 
☊空洞 ** D
防空洞* *
Definitions from Unihan Copyright © 1991-2015 Unicode® Inc. All rights reserved.
防 defend / prevent / embankment
空 empty, hollow, bare, deserted
洞 cave, grotto / ravine / hole
Hsk Characters:
HSK -Characters and beyond:
| Gb | Big5 | Eng | Rad | Mega | Triple | Multilingual |
|---|
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Hohlwelle (S)Kaverne (S)schalen (V)hohl (Adj) [kong1 dong4] 
空洞
Luftschutzbunker (Mil) [fang2 kong1 dong4] 防空洞
nichtssagend, ohne Substanz (S)nichtig (Adj) [kong1 dong4 wu2 wu4] 空洞无物
Ozonloch (S) [chou4 yang3 ceng2 kong1 dong4] 臭氧层空洞
Ozonloch (S) [chou4 yang3 kong1 dong4] 臭氧空洞
Theorie der hohlen Erde (S) [di4 qiu2 kong1 dong4 shuo1] 地球空洞说
