Hah! That took me a while too! I cleared the whole thing, but couldn't find it.
I would like to tease you some more about the location, but ah well, here goes: It's right next to the Oracle. There's a small passage about 20 meters from her face, before you even head into the big room with all the web passages etc.
I took the web passage, and got lost for an hour till I realized it was the other cave. At least I found some hidden treasure in the other direction so it was worth it.