Ring Around a Tree is a spiral treehouse built around a 50-year-old Zelkova tree by Tezuka Architects at the Fuji Kindergarten in Japan. Wood and transparent glass. 

The Minister’s Tree House, Crossville (Cumberland Co.), Tennessee. 

The world’s tallest tree house, built by Horace Burgess (who had a vision from the Lord), using tons of reclaimed wood. It is now apparently abandoned, yet there are still posts by folks who have visited and explored the ten-story-tall structure (100 feet into the treetops):  ”I whispered a prayer that building code inspectors never happen out this way. Amen.”