Tolkien não é nada específico:
'Long time I fell,' he [Gandalf] said at last, slowly, as if thinking back with difficulty. 'Long I fell, and he [the Balrog] fell with me. His fire was about me. I was burned. Then we plunged into the deep water and all was dark. Cold it was as the tide of death: almost it froze my heart.'
'Deep is the abyss that is spanned by Durin's Bridge, and none has measured it,' said Gimli.
'Yet it has a bottom, beyond light and knowledge,' said Gandalf. 'Thither I came at last, to the uttermost foundations of stone. He was with me still. His fire was quenched, but now he was a thing of slime, stronger than a strangling snake.
'We fought far under the living earth, where time is not counted. Ever he clutched me, and ever I hewed him, till at last he fled into dark tunnels. They were not made by Durin's folk, Gimli son of Glóin. Far, far below the deepest delving of the Dwarves, the world is gnawed by nameless things. Even Sauron knows them not. They are older than he. Now I have walked there, but I will bring no report to darken the light of day. In that despair my enemy was my only hope, and I pursued him, clutching at his heel. Thus he brought me back at last to the secret ways of Khazad-dûm: too well he knew them all. Ever up now we went, until we came to the Endless Stair.'
Portanto, sabemos apenas que Gandalf caiu por um "longo tempo" para os "fundamentos mais extremos da pedra", um lugar "além da luz e do conhecimento" que estava "muito, muito abaixo da mais profunda investigação dos Anões".
Contudo, a velocidade terminal de um corpo humano é de cerca de 55 metro / s, não de 700; portanto, mesmo que o palpite cinemático escolhido por valor dramático de Jackson esteja correto, estamos "apenas" conversando alguns quilômetros.
(E se você quiser se tornar exigente, dois outros pontos: primeiro, a queda foi medida a partir de um ponto de partida dentro das Montanhas Nebulosas. Eles podem ter começado um quilômetro acima do nível do mar, pelo que sabemos, e assim não estar muito fundo na crosta. Segundo, se eles começassem perto do nível do mar, depois de caírem no primeiro quilômetro, a densidade do ar excederia a do nível do mar e a velocidade terminal diminuiria um pouco.)