It's right under the Mara statue. I once said the water is coming from between her teeth, but I was mistaken. There's another pit under Mara's mouth where the water comes from and pours down & out into the lava pit.
But as I mentioned, it was nowhere near as big as what I saw on opening weekend. As of now, it's hardly even noticeable Opening weekend, I clearly heard it and that's why I looked down. This last weekend, I couldn't even pinpoint where it was anymore, but I did see it, and I saw the water, but there was so little of it, I can't even really qualify it as a waterfall.