 The cathedral , along with the rest of the city , has been sinking into the <unk> from the day it was built . However , the fact that the city is a <unk> with over 18 million people drawing water from underground sources has caused water tables to drop , and the sinking to accelerate during the latter half of the 20th century . Sections of the complex such as the cathedral and the tabernacle were still sinking at different rates , and the bell towers were tilting dangerously despite work done in the 1970s . For this reason , the cathedral was included in the 1998 World Monuments Watch by the World Monuments Fund . 
