So Earth has just suffered a horrible catastrophe and most of civilization is back in a Middle Ages level of development. A few people live on other planets relatively unscathed, with highly advanced technology (But nothing inconceivable. No teleporters, no going faster than the speed of light, and computing never got past basic optical processors, so it's only a few orders of magnitude faster than the current fastest supercomputer.) They have decided not to try to let people on Earth gain technology because they are afraid that they will lack the ability to use it right.
Unfortunately, the climate is completely unsustainable and requires maintenance to prevent it from spinning completely out of control. Like CO2 levels at brinkpoint, weird changes in climate, etc. Their solution is to put a refinery on the planet where they use large amounts of energy (they have large amounts of energy) to collect CO2, CH4, and other atmospheric pollutants and re-release harmless O2 and H2O.
However, I can't think of a way to put this on Earth's surface without allowing the inhabitants to know. I would like not to have to set aside an entire continent.
My first idea was radiation (covering only the boundary / the plant is automated and can handle radiation), but that has a tendency to spread. I was wondering if there was any way to create a barrier that would prevent middle age-level technologically advanced people from entering. (I'm thinking they have no gunpowder, no animals (all killed off in disaster), and little wood because of the climate's effect on forests. So not that much of a threat, just don't want them snooping around and getting advanced technology.)