Related Clues:
- Allotrope of oxygen
- Less common form of oxygen
- Gas (in a layer?)
- Main constituent of natural gas
- (Depleted) layer of gas
- Marsh gas — the name (anag)
- Gas forming protective layer
- Gas formed from oxygen
- Complicate the name used for natural gas, for the most part
- Lightning by-product
- O-O-O
- Layer that filters out UV rays
- Lightning strike by-product
- Australian has one for hole in Sky
- With 40-Down, depleted atmospheric region