I thought that Brazil just submitted all of their stadiums over 40,000 and that was were the 18 came from. Here's a report I found that mentions their stadium situation. http://ukpress.google.com/article/ALeqM5g1ne43ZsM0cZ2JKWg1XPWFCOzeqw It looks like regardless of how many they use there will be a major cost to go with it.
That is what I was going to say - exactly on December 21, at least according to the Mayans, and maybe even Einstein and Nostradamus!
