On a blog called Fogonazos, someone has compiled photos of the overflow infall of Monticello Dam in California, calling it the largest drain hole in the world. There are also photos of other similar structures elsewhere in the world. Once you stop giggling about them being called glory holes, it's an interesting collection.
However, they seem to completely ignore the Pothole and the Funnel, the great big one in Australia. Which looks much bigger than the Californian one in photos, like this one.