are distinguished from conventional stadiums by their enclosing roofs. Many of these are not actually domes in the pure architectural sense, some being better described as vaults, some having truss-supported roofs and others having more exotic designs such as a tensegrity structure. But, in the context of sports stadiums, the term "dome" has become standard for all covered stadiums, particularly because the first such enclosed stadium, the Houston Astrodome, was built with an actual dome-shaped roof. Some stadiums have partial roofs, and a few have even been designed to have moveable fields as part of the infrastructure. The Mercedes-Benz Superdome in New Orleans is a true dome structure made of a lamellar multi-ringed frame and has a diameter of 680 feet (210 m). It is the largest fixed domed structure in the world.