Since it was founded by Hernán Cortés, Mexico City, built upon the ancient Aztec lake city of Tenochtitlan and now the third most populous metropolis in the world, has continued to sink into the ground. During the second half of the 20th century, the exploitation of the groundwater to meet the needs of 18 million people has also dried the soil, only accelerating the sinking phenomenon. After the Spanish conquistadors conquered the Aztec Empire, they began to consolidate Spanish power in the New World, often by building on the ancient sites of Aztec temples, sometimes even reusing stones from the destroyed temples to build new churches. Work is currently underway to slow and stabilize the sinking of several buildings in the city. The San Francisco church, the original headquarters of the first twelve Franciscan monks sent by the Pope to evangelize New Spain, is now under its third reconstruction. Despite these efforts, the church keeps sinking, and one must now descend a staircase up to 7 ft (213.36 cm) in length to reach the door of the building.