Sagrada Família, Barcelona

Gaudí's unfinished basilica is the most ambitious architectural project in history. Since construction began in 1882, it has set a record for longevity in progress, with current estimates targeting completion in 2026—after 144 years. The temple's intricate facades and hyperboloid vaults surpass the structural complexity of any Gothic cathedral; for example, the central tower will reach 172.5 meters, just 0.5 meters shorter than Montjuïc's hill to respect Gaudí's reverence for nature.