§ Of Tuor and the Fall of Gondolin The Silmarillion

Then the days of Gondolin were yet full of joy and peace; and none knew that the region wherein the Hidden Kingdom lay had been at last revealed to Morgoth by the cries of Húrin, when standing in the wilderness beyond the Encircling Mountains and finding no entrance he called on Turgon in despair. Thereafter the thought of Morgoth was bent unceasing on the mountainous land between Anach and the upper waters of Sirion, whither his servants had never passed; yet still no spy or creature out of Angband could come there because of the vigilance of the eagles, and Morgoth was thwarted in the fulfilment of his designs. But Idril Celebrindal was wise and far-seeing, and her heart misgave her, and foreboding crept upon her spirit as a cloud. Therefore in that time she let prepare a secret way, that should lead down from the city and passing out beneath the surface of the plain issue far beyond the walls, northward of Amon Gwareth; and she contrived it that the work was known but to few, and no whisper of it came to Maeglin's ears.