It has taken 125 years to build so far, surviving the loss of the plans, a chronic lack of funds and the death of its creator, flattened by a passing tram.
But Barcelonaâ€™s world-famous cathedral, the Sagrada FamÃlia, could be facing its toughest challenge yet: the construction of a tunnel for high-speed trains next to its foundations. â€œWe are extremely concerned about the tunnel passing so close,â€ said Jordi Bonet, the cathedralâ€™s director, explaining that a protective wall would be built just 75cm (30in) from the foundations.
Mr Bonet, the chief architect who has spent 22 years trying to finish Antoni GaudÃâ€™s surreal masterpiece, says the planned excavations â€œcould prove fatalâ€ for the landmark and cause â€œirreversible damageâ€.
Giant tunnelling machines will bore a 12m (39ft) wide tunnel through the sandy, water-logged land under the cathedral, he said, risking subsidence or flooding.
Even if the cathedral survives the construction, Mr Bonet fears the vibrations from trains could open cracks in the building or shake tiles loose from the ceilings, which tower 75m above touristsâ€™ heads.
Several thousand local residents have also joined the campaign, fearing that their homes could be affected. â€œWe must protect the Sagrada FamÃlia,â€ they said in a recent statement, after forming a human chain around the cathedral. â€œWe cannot allow anyone to endanger GaudÃâ€™s jewel.â€
The cathedral is visited by an estimated 5 million tourists a year and has always provoked strong reactions. The writer George Orwell once berated 1930s anarchists for â€œshowing bad taste by not blowing it upâ€ during the Spanish Civil War. Rioters did, however, manage to set fire to GaudÃâ€™s intricate plans and models, forcing his successors to take some educated guesses on how to proceed.
Using computer-aided techniques, the present architects have greatly speeded up the construction: they hope to finish it in 30 years. Still, GaudÃ would not have worried about the delay. He was once said to have joked that his client â€” God â€” was in no hurry.
The rail link between Madrid and Barcelona is due to be completed by the end of this year, halving the time it takes to make the 370m (600km) journey between Spainâ€™s two largest cities to only 2Â½ hours.
The Government is adamant that the line must reach the heart of Barcelona. But it is this final leg, from Sants train station into the city centre, that has provoked such hostility. Without that final stretch, however, the Government fears the train will not be able to compete with the airline shuttle services on what is the worldâ€™s busiest air route.
The Government and Barcelonaâ€™s authorities say there is no risk to the cathedral or residents. They have carried out detailed studies and are building the concrete wall so close to the cathedral precisely to protect it. The regional government of Catalonia has also accused its critics of being alarmist and cautioned against creating further delays.
The International Council on Monuments and Sites, which advises Unesco on world heritage sites, has already spoken out against the tunnel. In a recent report it decided that the project â€œdid not offer sufficient guaranteesâ€ to proceed.
The Archbishop of Barcelona, LluÃs MartÃnez Sistach, has also called on the Government to change the tunnelâ€™s route.
Here's hoping the engineers have got their figures right....