ERZ3 is very cleverly hidden, but there's no clear telling that its entrance is even safe to go to. Some sort of clue would be very appreciated, rather than a leap of faith.
Pretty neat! Although I have to say, one thing that would look way better than those yellow floor numbers is some sort of actual zone signpost.
No, that's not the reason.
As you can see such hidden levels as
are present and fully accessable.Pipe Towers, Aerial Garden, Spring Hill and Azure Temple.