A Truly Gigantic Scientific Meeting -- AGU

I just learned last night that the numbers are still rising and now 19,200 people are expected to attend the AGU fall meeting here in San Francisco. And maybe by the end it will get to the magic number--20,000. That doesn't make it the biggest scientific conference in the world. The Society for Neuroscience annual meeting is considerably larger, though their numbers seem to be falling from a peak of nearly 35,000 in 2005. Still, the scale of the AGU event is pretty staggering. For geoscientists, it is definitely the place you have to be every year. Are there any other scientific meetings, besides AGU and Neuroscience, that get this large I wonder? P.S.: I should add, it is my understanding that this will be AGU's largest meeting ever, and thus also the largest geosciences meeting in the world...