Antoni Gaudí's Barcelona basilica, under construction since 1882. Expected to complete in 2026 — the 100th anniversary of Gaudí's death.
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 the 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".