First come, first serve for non ticket holders. They open doors at 10, lines was already formed when we arrived at 9.
Sometimes groups come and have extra tickets. If they give those to the guards the guards will pass them on to those standing by. It is not structured so you really have to be standing close outside the building and asking if anyone has extra tickets. I saw a lot of people get in that way while I was waiting for my group to arrive.
Try going online for same day passes. Be on by 0600/0615
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