system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are there airpockets in underwater caves that are clearly under sealevel?
gpt
The caves may have a hidden opening or cracks somewhere to allow air in. If not the air pockets may come from methane or carbon dioxide from degradation of organic matter on the sea floor or some kind of natural gas pocket. It's unlikely that any air pockets in an underwater cave would be breathable since sunlight would be needed for plants or algae to make oxygen. 

Another theory could be that the cave was above sea level at some point and the rising ocean could have caused air to be trapped in the cave.