" " The exterior of the unfinished Sagrada Familia Basilica is seen on July 13, 2019, in Barcelona, Spain. The building is a UNESCO World Heritage site. Patrick Gorski/NurPhoto via Getty Images

La Basílica de la Sagrada Familia ("The Basilica of the Holy Family") in Barcelona, Spain, is unlike any building in the world. The church's famous Nativity Facade looks from a distance like supersized drip sandcastle, but closer inspection reveals ornately sculpted towers that have been described as "the Bible written in stone."

The architect of the Sagrada Familia was the eccentric and deeply religious Catalan genius, Antoni Gaudí, who was struck and killed by a streetcar in 1926, when only the Nativity Facade of his masterwork had been completed. The rest of the ambitious structure existed only in Gaudí's complex architectural drawings and scale plaster models.

Tragically, Gaudí's priceless models and most of his drawings were destroyed by anarchist revolutionaries just 10 years after his death. And ever since, generations of architects and engineers have labored to piece together Gaudí's singular vision and finish his magnum opus.

As of this writing, the Sagrada Familia is 137 years old — the first stone was laid in 1882 — making it the longest-running active building project on Earth.

Incredibly, it wasn't until 2019 that the city of Barcelona officially granted Sagrada Familia a construction permit, even though Gaudí applied for one in 1885. The permit itself was the most expensive on record, a whopping $5.2 million (4.6 million euros) to complete the monumental project by 2026, the 100th anniversary of Gaudí's death.



A Forest of Stone

As an architect and structural engineer, Gaudí was at least a century ahead of his time. His 11 visionary works in and around Barcelona — seven of which are UNESCO World Heritage Sites — demonstrate how Gaudí employed wildly innovative building techniques to mimic the beauty of nature.

Gaudí's Park Güell, on a hill overlooking Barcelona, features a broad patio contained by wavy, curved walls adorned with colorful Mediterranean-tiled mosaics. And his Casa Battló looks as if an alien apartment complex landed in the middle of a 19th-century city. Its breathtaking facade is an architectural crazy quilt of curved stone, iron, glass and tile, all girded by Gaudí's uncanny grasp of engineering and materials science.

" " Architect Antoni Gaudí (1852-1926) thought that buildings could be constructed like sculptures. Apic/Getty Images

Gaudí was a devout and passionate Catholic who believed that the deepest way to commune with God was to abide in nature, his creation. When 31-year-old Gaudí received the commission to build the Sagrada Familia church, he drew up plans for a towering structure with 18 spires representing the 12 Apostles, four evangelists, the Virgin Mary and Jesus Christ.

In contrast with the grand exterior, the inside of the Sagrada Familia is meant to evoke a naturalistic forest of stone. Massive pillars stretch from floor to ceiling, where their twisting branches intertwine to form an ornate canopy. Work was slow due to the technical complexities of the cathedral and the fact that it relied solely on private donations.



Getting Inside Gaudí's Mind

Gaudí worked on the Sagrada Familia for 40 years, fully devoting the last years of his life to the project. "He wanted to write the history of the Catholic faith in one building," biographer, Gijs van Hensbergen, told the TV show "60 Minutes." Van Hensbergen added that Gaudí slept at the construction site and took little care for appearance, wearing frayed pants held up by a rope belt.

Gaudí knew that he wouldn't live to complete his masterwork, so he drew up detailed architectural plans, and most importantly, scale plaster models of each of the cathedral's sculpted facades and towering spires. When Gaudí was killed in the 1926 tram accident, his associates were able to keep building based on the architect's drawings and models.

But then the Spanish Civil War erupted in 1936, and gangs of anarchists targeted churches as symbols of institutional oppression. The Sagrada Familia was attacked, and although the structure was miraculously saved, the anarchists broke into the architects' workshop and smashed Gaudí's precious plaster models.

Without the plaster models, progress on Sagrada Familia slowed to a crawl, further delayed by the decades-long fascist rule of Francisco Franco. When New Zealand-born architecture student Mark Burry backpacked through Spain in the 1970s, he was recruited to the cause of finishing Sagrada Familia while remaining true to Gaudí's original vision.

" " An interior view of the La Sagrada Familia Roman Catholic church on Feb. 6, 2019, in Barcelona, Spain. Construction started in 1874 and was designed by Catalan architect Antoni Gaudí. Matthew Horwood/Getty Images

When Burry asked how the stone masons and other craftsmen were receiving their instructions, he was shown boxes and boxes containing the fragmented remains of Gaudí's plaster models.

"My job was to reverse engineer the models," Burry told "60 Minutes." It was the only way to faithfully reconstruct the vision locked inside Gaudí's singular mind.



Technology to the Rescue

Burry worked on Sagrada Familia for 30 years and is responsible for bringing the project into the 21st century. He consulted with industrial designers working on vehicles, ships and other complex engineering problems. All of them were using 3-D aeronautical drafting software that could prototype designs and materials digitally before constructing them in the real world.

So Burry decided to employ the same drafting technology to solve Sagrada Familia's thorniest engineering challenges on computers before casting them in concrete, iron and stone.

For a 137-year-old construction project, today's Sagrada Familia is remarkably high-tech. In addition to using the latest virtual modeling software, components are prototyped using 3-D printers, and architects can explore and interact with digital models using virtual reality.

Because of these technological and engineering advances, the current chief architect of the Sagrada Familia, Jordi Faulí, is confident that Gaudí's grandiose structure — including the tallest church spire in the world at 566 feet (173 meters) — will finally be completed in 2026, the 100th anniversary of Gaudí's death.

At least that's the hope. As Faulí mused to Architect Magazine in 2014, "Are the great cathedrals and basilicas of the world ever truly finished?"

(Note: The time-lapse video below shows how the cathedral will look when it's finished.)