What is it called when a diver thinks they see an air bubble but it is really just a different level of water?

I remember watching cave diving videos on national geographic, and I really want to know what this is. It said that sometimes the cave water will be separated from each other since they are different types of water, which can trick the diver into thinking it is an air bubble when in reality it's just a different level of water. Does anyone know what that is called?