Antoni Gaudí's still-unfinished Barcelona basilica, begun 1882 and projected to complete in 2026 — the 144-year cathedral.
From Wikipedia
Basílica i Temple Expiatori de la Sagrada Família, or simply Sagrada Família, is a Catholic church under construction in the Eixample district of Barcelona, Catalonia, Spain. Designed by Catalan architect Antoni Gaudí, it is the tallest church in the world. On 7 November 2010, Pope Benedict XVI consecrated the church and proclaimed it a basilica. Parts of the Sagrada Família were declared as an UNESCO World Heritage Site in 2005, under "Works of Antoni Gaudí".