Amazing TreeHouses architecture around the world

Remember the good old days when a tree house was a rough-looking assemblage of two-by-fours and plywood wedged into the crook of your backyard oak tree?

These days, tree houses are more than just hideaways for the Bart Simpsons of the world. They represent a creative challenge to visionary architects in every corner of the globe. Firms like Germany's baumraum, which specializes in offbeat projects, and designers like Pete Nelson, a Washington State-based genius of the form, have made it their mission to bring this humble concept into the future.

They fuse the natural world with the artificial in ways that are often stunning: spheres suspended in British Columbia's Pacific rain forest; a "mirror cube" that looks like a woodsy warp in the space-time continuum; a sleek, modern cabin overlooking the German countryside, set in the 'V' formed by opposite-leaning oak and alder trees.