Metropolitan Cathedral

Before going into the Cathedral, take a good look at it: it isn't straight. Built on the bed of a drained lake and on the remains of Mexica temples, it has been sinking unevenly for centuries. By the end of the 20th century, the difference in level between its highest and lowest points was around two and a half metres.

1A building on a lake

The clay of the old lake compacts as water is pumped out of the subsoil, and the enormous weight of the church did the rest. In 1993 an extraordinary engineering project began, known as under-excavation: shafts were dug beneath the building to remove clay from under the areas that were sinking least, so that the whole structure would settle more evenly. The aim was not to straighten it completely but to recover part of the distortion and stop the cracking. Inside, a plumb line hanging in the nave lets you see the tilt with the naked eye.

2The Christ who absorbed the poison

Inside, a dark crucifix known as the Lord of the Poison is venerated. According to tradition, a devout man kissed the feet of the image every day, and an enemy smeared them with poison to kill him. When the worshipper leaned in, the Christ supposedly absorbed the poison and turned black, and the man was saved. The image used to be venerated in the church of Porta Coeli, a few streets away, and came to the Cathedral in the 20th century.

The engineering is documented to the centimetre; the miracle of the poison belongs to popular devotion.

3What to look for

• The line of the façade: compare it with the neighbouring buildings and you'll see the tilt.
• The repaired cracks in the walls and the steps of the forecourt.
• The plumb line beneath the vaults, if you go inside.