Camp Century wasn't a secret the way you're implying. The Wikipedia page on it was created in 2019: https://en.wikipedia.org/wiki/Camp_Century

It's been publicly known since 1996 when the documents on it were declassified. The Danish government was informed about it prior to construction, and the Danish government already had an agreement with the US to allow military bases to be built on certain areas of Greenland. The Danish government did not explicitly approve of Camp Century, which was built outside the agreed upon areas. That's an interesting story: https://www.sciencenordic.com/denmark-forskerzonen-history/how-the-us-built-a-mysterious-military-camp-under-the-greenland-ice-sheet/1451993