The fissure was in Beleriand, but that is all we can tell. That area was ruined by the War of Wrath, and sunk beneath the sea, so I guess that fissures were opening up every where. To help provide some information and evidence I will use my perennial source of information, and guide to middle earth, the Encyclopedia of Arda.
War Of Wrath
Quote:
After Eärendil came to Valinor to beg for the aid of the Valar, they sent a mighty force into Middle-earth to do battle with Morgoth. Led by Eönwë, the herald of Manwë, the forces of the Valar, with the peoples of the Vanyar and the Noldor that had remained in Aman, sailed eastward in the ships of the Teleri.
In the great battle that followed, Morgoth's armies were almost completely destroyed, and only a few of his Balrogs, Dragons and Orcs survived. Morgoth himself was taken as prisoner and shut beyond the confines of the World, and the Silmarils were recovered from his Iron Crown. In the tumult of the war, Beleriand and the lands to the north were destroyed and sunk beneath the sea, and the shape of Middle-earth was changed.
In the great battle that followed, Morgoth's armies were almost completely destroyed, and only a few of his Balrogs, Dragons and Orcs survived. Morgoth himself was taken as prisoner and shut beyond the confines of the World, and the Silmarils were recovered from his Iron Crown. In the tumult of the war, Beleriand and the lands to the north were destroyed and sunk beneath the sea, and the shape of Middle-earth was changed.
The key bit is the last sentence, ssaying that the world changed, it is unlikely that part of the world was 'stable' at that time.
Maedhros
Quote:
After the defeat of Morgoth in the War of Wrath, Maedhros and his brother Maglor stole the two remaining Silmarils from the camp of the Valar, but the hallowed Jewels would not suffer the brothers to hold them. In torment, Maedhros cast himself into a fissure in the earth.
So we can assume that fissures were opening up, and Maedhros threw himself into one. Hopwever I think that it does not matter 'where this fissure was. It is not important
And I hope that helped you a little.
