Other Anthem Guides:
All Hidden Places Locations Guide (Explorer Challenges)
There are a total of 10 Hidden Places that you need to find to complete all of the "Explorer" challenges in Anthem. Each explorer challenge / trophy asks you to find all districts, landmarks and hidden places in each of the regions. This video covers all the hidden places.
The Hidden Places are basically mini dungeons, they contain enemies as well as chests to loot.
For some reason, there are a few hidden places on the map which do not count towards any challenge. Why this is, I don't know, but I have decided not to include them to avoid any confusion with the challenges.
None of the hidden places are missable and you can find them at any time. All you have to do is enter the hidden place in order for it to tick off as complete in the specific explorer challenge for that zone.