The world's largest treehouse is in Crossville, Tennessee. It's a constant work in progress by Horace Burgess. It's no longer open to the public since the state closed it down in 2012 due to fire code regulations, but Horace is still adding to it. The main tree is an oak tree with a 12 foot diameter trunk, and there are six other oak trees that help support the house. Google Maps has it listed as "The Minister's Tree House".