: Bungie talked about it during the commentary they did for Halo 2's cutscenes.
: That cutscene came after was was originally from the Forerunner Tank level
: where the tentacles were to be shown crashing though levels and walls and
: be a bigger part of gameplay. A lot of the cutscenes were made before the
: levels were cut from the game.

I remember a big fight on here right after halo 2 came out, with some claiming tentacle, and others claiming cable. There is a lot of busted machinery in that room, so i had always assumed that there had been a battle between the gravemind and the sentinels/enforcers, and that the tentacle was a sliced off piece. The gravemind could have been pushed back down into the hole, or left when the covenant fleet/high charity showed up at the temple.