The Beijing National Stadium, a.k.a. "the bird's nest" due to the "web of twisting steel sections that form the roof," was designed by Herzog & De Meuron Architekten, Arup Sport and the China Architecture Design and Research Group. With a gross volume of 3 million cubic meters, it is considered the largest enclosed space in the world, and with 26 kilometers of steel, it is also the world's largest steel structure.
Image source: Thousand Wonders