Mexico City Metropolitan Cathedral (Catedral Metropolitana)
Within its fantastic bulk are sheltered some 16 chapels, several alters and retablos, a fine parish church, and a choir, each an inspired work of art replete with gold gilt, fine paintings, and sculptural details. Above it all, 25 bells - measured in tons - ring and sing to the city all around.
The cathedral sits atop a far more ancient foundation, originally built for a temple to Huitzilopochtli, the Aztec God of war. His pyramid was destroyed by the Spanish, who built their cathedral in its stead. As the thirsty city emptied its underground aquifer, however, this massive monument began to sink into the subsoil. It was stabilized in the 1990s, but still lists to one side.
All roads lead to the Zócalo, but you're much better off without a car - traffic is terrible. Instead, take Metro Line 2 to the Zócalo stop, right in front of the cathedral.