What does 'air passing' in context of drilling a borewell means?

I would like to understand what does air passing mean. We have drilled a borewell in Bangalore. We have got the water source at 850 ft and tried to drill it further. When we reached 875 ft, driller told us that it cannot be drilled further as there is air passing in the borewell. Please help to understand about "air passing" and suggest a better water source.

Thank you.

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