The Prado alone ranks with the world's top art museums for the riches of its collections. But add the Reina Sofia National Art Museum, the Thyssen-Bornemisza Museum, and the CaixaForum, all along Madrid's mile-long, tree-shaded boulevard, and you have what may be the world's highest concentration of priceless art treasures. It's no wonder this is known as El Paseo del Arte - Boulevard of the Arts.