What is between the two layers of glass?

In short- there is no vacuum between the two layers of glasses, it is a layer of air. 

The holes vent the hot air to prevent the glasses from exploding since the glass expands when heated. The divot's added to help equalize the air pressure when the glasses are cooling down. The holes are then sealed once the glasses have cooled with a food-safe UV cured resin. 

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