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Atmospheric pressure at sea level is about 10.2 tonnes per square meter, or about 100kN of force on a square meter. In "Physics Land" (ie everything behaves in an ideal manner), assume that you have a square tube with a 1m x 1m hole, open at one end, and a perfectly-fitting square plug that can move along the square tube. To produce a cubic meter of vacuum, you just need to pull the plug out by 1 meter.Now, Energy=Force x Distance=100x103N x 1m = 100kJ.Of course, as other respondents have noted, there is no such thing as a perfect seal, moving a plug is likely to scrape off atoms (and larger lumps), and there is no material which has zero vapour pressure. So the above is a "best possible" answer.