: 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.

It's a tentacle according to the model/texture.

If it's not common knowledge already, the Index area shaft the Arbiter is tossed down is directly above the chamber we see the Gravemind in, so the Arbiter had a straight drop down to where he was caught, but the interesting part is the room itself that the Gravemind is in is an almost exact reuse of the Halo 1 Control Room model, with only a minor modification of the platform leading from the wall and the absence of the Marathon-esque circular walkway.