How to fill with air

I am creating a game called Water World, and I am making an underwater base. I am trying to make it so that you can walk into it after swimming down to it, but it won’t fill with air.

If you’re trying to erode or subtract the water (which I assume is terrain), then make sure Ignore Water is ticked off- you could also select and delete.

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Ok, thanks for the advice! I hope it works!