So far the most known secret, is the secret room. I do not have pictures of it, but have found and been in the room myself. Reports say that this room is in AR #1 and you must clear out the area under the elevator and you'll find a tunnel. This tunnel should have the 'E' button prompt and say something like "Unknown" or "Secret" location. You venture in and I found there to be level one Gooeys and one chest that gave me llama fluff. Uneventful. There are folks who have spent weeks in game clearing out the mine and have not found it, while others have stumbled on it by accident. We don't know if it always shows up or is a random spawn, but there you go. Also in the AR #1 is a large piece of architecture with a hit box (this one I haven't found). In order to properly mine it, you need to upgrade your pick axe to bronze or higher. From what I heard, this is also uneventful when you break out.