Why is Apple building a large structure for it's Sep 9th event?

Today Apple announced that its next event is to be held on September 9th, which is likely include the announcement of two new iPhones and a rumoured iWatch. Apple is hosting the event at Flint Center for the Performing Arts at De Anza College in Cupertino, the same place Steve Jobs introduced the original Mac and the blue iMac which turned Apple's fortunes around. Things have got a little more interesting though as MacRumors has posted the above image sent in by a user which shows a giant structure at the Flint Center which is protected by security. This could be part of what Apple is set to announce, potentially an iWatch with some sort of home automation technology. We really don't know, but we can't wait to find out.