In the middle of the city's historic centre is the enormous paved Plaza de la Constitucion, or Zocalo, the second largest city square in the world, and Mexico City's centre of government and religion. The Presidential Palace dominates one side of the square, a magnificent colonial building that was built on the site of the former Aztec Palace, with remarkable interior murals narrating the story of Mexico's history. Dominating an adjacent side of the square is the great Metropolitan Cathedral, displaying a wealth of architectural styles and occupying the site of the once sacred grounds of the Aztec. The ornate interior contains its chief treasure, the King's Chapel and gilded altar. The Cathedral is one of the buildings subsiding into the soft ground on which the city is built and builders are continuously at work to prevent its uneven descent. The square itself is filled with activity, with vendors and buskers, informal traditional Aztec dance performances, family groups, workers on lunch break and passing tourists. It is also the place for demonstrations, government rallies and protest marches, as well as festivals and public holiday events. Every evening the presidential guards, in a show of great ceremony, lower the national flag from the central flagpole. And encircling the square is the continuous buzz of the ubiquitous green Volkswagen taxis.