[XPhial]

Just to be literary for a moment:

Since Mt. Doom is a volcano, and a volcano is simply filled with molten earth, the only thing that could detroy the Ring was Middle Earth itself. It's one more example of how Tolkien uses the very land as a character. Now, Mt Doom is a specific volcano, and likely the only active one in Middle Earth. The question remains, was it natually active or manipulated, like so many other things, by the will of Sauron? If it was manipulated, then it's likely that even another volcano would not have been sufficient to destroy the ring...only the cradle of its forging. Since that question remains unanswered, however, we can speculate that only with the characters' cooperation with Middle Earth itself could the ring be defeated.