No doubt you have heard of it and seen some images over your lifetime, and I have been looking in on its construction in visits for fifty of my 72 years, but the construction is now accelerating towards completion and the structure is no longer merely amazing but has become gobsmacking astonishing.
When a few days ago I emerged from the nearby rapid transit station and saw it again for the first time in ten years my jaw dropped and I froze, and all around me almost everyone else in the throngs reacted the same way. I have only seen such reaction to the great Pyramids of Giza and events such as once when a huge adult gray whale suddenly blasted out from the ocean near to my boat, splashing back down in an impossible fantastic display of power, almost swamping us.
The size is amazing, but many edifices are much taller or larger. However the architecture of the Sagrada Familia embues every cubic meter of the building with luminous power. The lines of the towers of the building as well as of the nave and apses are catenaries echoing the structures of nature, not man. The linked article above discusses this and the building’s amazing history, but here is a list of articles going into the subject of the underlying mathematics in greater depth:
The architect, Antoni Gaudi, was a brilliant man at the level of Leonardo da Vinci, a superb mathematician, engineer, mystic, and manager.
If you are going anywhere near Barcelona, make sure to go and see the Sagrada Familia. Before traveling make reservations to visit well in advance on line, as it is now almost impossible to gain entry otherwise.