"The entrance shall only be found under a cloudless rain" was a clue in the letter from Queen Victoria to Albert Pike.
History
When Ben, his parents, Riley Poole, Abigail Chase, and Mitchell Wilkinson are looking for the entrance to the Cíbola at the Black Hills of Custer State Park, Mitch reveals the clue. Ben figures out that water change the shading of the rocks. This reveals an engraved eagle leading to a gap in the rocks.