A balloon does a bloody good job of trapping gas, and so does the paper thin aluminium of an aircraft at altitude, or the tin cans of space stations, or an even

A balloon does a bloody good job of trapping gas, and so does the paper thin aluminium of an aircraft at altitude, or the tin cans of space stations, or an even better example, a bigelow space station. why do you find it so hard to believe a denser material like rock would be able to? I'm sure you're concerned about fissures and other gas leaks, but 5 metres of most rock is going to do a pretty bloody good job, and I don't think anyone is saying we are going to be living in raw rock hewn caves - putty up the cracks or line with some other material and you're done