What secrets do you think the government are hiding from us?

Now im not a conspiracy theorist and i don't believe in little green men but you only have to look through history to see that the government have always hidden information from us. For example in the the late 50's the British government built a 35 acre subterranean city with 60 miles of roads, a hospital,kitchens,canteens,offices,studi… and even a lake, plus room to accommodate up to 4000 people yet no one even knew it existed until 30 years later. Makes you think if they managed to hide something on this scale from us for 30 years what could they be hiding from us right now?