there's allot involved building a safe under ground structure. with that size you need a bobcat to dig that. not a good idea go with a few larger plants or numbers next outdoor season
i like the idea of an underground grow bunker, just needs to be on your own land, then that shit will rock. what ever happened to the guy who was digging out a room under his house?
anyway a secret bunker with a hidden way in would be so cool, but you will have to go for a sealed room as the exhausting hot air would be impossible to keep hidden, while the air exchange from an air conditioner is a lot less in volume easier to have that air gently wafting up through a pile of wood or something to hide the vent. but the one thing you will have is a hell of a lot of work to get to the point when you can put the first seed in earth. hehe.......
Underground Growroom...crazy??