Nurtalë Valinóreva, Quenya for the "Hiding of Valinor", was the time and action taken by the Valar to render Valinor unassailable by Morgoth. It was prompted by Morgoth's attack upon Tilion, the Maia charged with guidance of the Moon. Shadowy spirits had been sent against Tilion and there had been a battle in Ilmen that Tilion had won. But although Morgoth had failed in this assault the Valar feared the damage he might yet contrive against Valinor.
To permanently thwart any attack they caused the Pelóri to rise up to new and dreadful heights, with outer walls as smooth as glass. In the Calacirya, the only pass permitted through the mountains, strong towers were set. In the ocean the Enchanted Isles were strewn in the Shadowy Seas north, east, and south of Tol Eressëa. In these waters were dark rocks wrapped in mist, and in the twilight any mariner would grow weary and loath the sea. If a sailor reached one of the isles he would sleep until the world was changed.