 The sinking ground and seismic activity of the area have had an effect on the cathedral 's construction and current appearance . Forty @-@ two years were required simply to lay its foundation when it was first built , because even then the Spaniards recognized the danger of constructing such a huge monument in soft soil . However , for political reasons , much , but not all , of the cathedral was built over the remains of pre @-@ Hispanic structures , leading to uneven foundation from the beginning . 
