The Pendrath na-Udûn (S. "Stairs to Hell;" La. Kivinen Selkäranka) formed a winding, stone stairway which climbed the smoldering heights of Naur Firnen. The stairs were carved into the contours of the mountainside long before the memory of Men, following a sometimes open, sometimes secretive line up the ridges and valleys of the Naur Firnen. Built to serve as an outpost of Angband in the days before the War of Wrath, these stairs remained as silent witness to Morgoth's passing. In several places the steps were narrow, steep and difficult to scale for those heavily encumbered with packs and equipment; in other places, they were blocked by stone-falls and rock-slides, and a way had ti be cleared or an alternate path taken. The stairs climbed high up the mountain to a flat, open shelf of rock before the gaping mouth of a cave. In ancient times, the cave was called Doom's Door, and its mouth was guarded by a Balrog. But for all its terrifying names, little remained, its horrors either having been swept away or taken flight into the Underdeeps at Morgoth's downfall.
Beyond the door lay a great labyrinth of volcanic caverns. The caves were rough-walled and lifeless, save for an occasional Troll or wandering dragon. These caves gradually led eastward and ever downward until the roots of the mountain were reached. Here the air was warmer, but fouled by sulfuric fumes and other poisonous vapors. As the caverns dived ever deeper, it grew increasingly hot and the way forward more difficult as paths were blocked by ribbons of lava, snaking across tunnels and through caves. These were part of the outlying borders of the Lake of Fire beneath the Naur Firnen.
The molten rock here glew white hot, endlessly bubbling and steaming; but there was little chance of an eruption ever again reaching the world above. Outlying channels of the Lake of Fire had broken through to the great, subterranean roads of the Underdeeps, which siphoned off much of the pressure generated by the lake. Thus, the titanic forces that had once sent molten lava spewing out onto Angband's Threshold and the Fire Tundra had been spent before they started and could no longer build up sufficient strength to force the lava upward.