Why are the minarets of the Taj Mahal tilted outwards?

The four minarets surrounding the Taj Mahal in Agra are designed with a slight outward lean, to protect the Taj in case they collapse. The tilt seeks to ensure that the minarets will fall away from the tomb in case of calamities such as an earthquake. Additionally, the minarets' tilt provides an optical illusion, making the pillars look perfectly vertical.