§ Of the Beginning of Days The Silmarillion

Now Melkor began the delving and building of a vast fortress, deep under Earth, beneath dark mountains where the beams of Illuin were cold and dim. That stronghold was named Utumno. And though the Valar knew naught of it as yet, nonetheless the evil of Melkor and the blight of his hatred flowed out thence, and the Spring of Arda was marred. Green things fell sick and rotted, and rivers were choked with weeds and slime, and fens were made, rank and poisonous, the breeding place of flies; and forests grew dark and perilous, the haunts of fear; and beasts became monsters of horn and ivory and dyed the earth with blood. Then the Valar knew indeed that Melkor was at work again, and they sought for his hiding place. But Melkor, trusting in the strength of Utumno and the might of his servants, came forth suddenly to war, and struck the first blow, ere the Valar were prepared; and he assailed the lights of Illuin and Ormal, and cast down their pillars and broke their lamps. In the overthrow of the mighty pillars lands were broken and seas arose in tumult; and when the lamps were spilled destroying flame was poured out over the Earth. And the shape of Arda and the symmetry of its waters and its lands was marred in that time, so that the first designs of the Valar were never after restored.