Oak Island Base
How do you keep a secret? By not keeping anybody around who can see it. Oak Island is a small island in the Lake of the Woods that was ceded to the United States by the Webster-Ashburton Treaty, ostensibly to resolve surveying errors but mostly because the British government preferred that the underground derelict alien base found on it become the headache of somebody else. British savants had prodded and poked at the stupid thing for forty years, they got nothing out of it, and perhaps Brother Jonathan would have better luck. Or else get themselves blown up by randomly touching things.
The American government — with, admittedly, a lot of help from first the British, then the Canadian governments — has been themselves nervously poking at the place ever since. Oak Island Base is an extensive underground facility with ten levels, and enough space to host about five hundred humans; based on the size of the ‘beds’ and ‘tables’ in the facility, it was capable of hosting about seven hundred or so of the aliens. Most of the facility is empty: the aliens apparently took everything with them that could be easily removed. It wasn’t until the 1930s that researchers realized that there was an electrical system in the place, and the 1960s before somebody figured out how to refuel the nuclear reactor that powered it. Discovering that there were embedded video screens to switch on didn’t occur until 1998; people have been trying to decypher the digital menus and library ever since.