The video: The tallest structure in the United States — and the second-tallest in the world — is being planned for the desert outside Phoenix. The tower is part of a massive solar energy plant that will provide electricity for 150,000 homes when it's completed in 2015. (View video below.) At twice the height of the Empire State Building, the tower is essentially a chimney with 32 turbines inside. The 2,600-foot tall edifice is connected to a ground-level greenhouse that's even larger, at over two miles wide. When the air inside the greenhouse gets warm, it rushes toward the chimney and rises, turning the turbine blades and generating electricity. This process will continue even at night, thanks to residual heat rising from the ground. Unlike most electricity-generating plants, the solar plant burns no fuel, so creates no greenhouse gases.

The reaction: It's "gargantuan," says Paul Bentley in the Daily Mail. The tower is just a hundred feet shorter than the Buri Khalifa skyscraper in Dubai, currently the tallest building in the world. Though the $750 million project is far from finished, the Southern California Public Power Authority recently signed a 30-year purchasing agreement with EnviroMission, the Australian company that's spearheading the project. With a payback period of just 11 years for a structure that's designed to last some 80 years, this could be the prototype for other solar energy projects worldwide. "It's incredibly benign," says EnviroMission project director Chris Davey, as quoted by "No water, no dangerously high temperatures, no 'death rays' from mirrors, very few moving parts." Watch a video about the project: