On an average summer day in Paris, about 50,000 tourists pass through Notre Dame cathedral, one of the finest buildings of the medieval era still standing. Visitors from dozens of countries gaze up at the spectacular stained-glass windows, tiptoe through its vast choir and nave and whisper in awe at the centuries-old sculptures and paintings that line the walls.

Notre Dame, which looms over the capital from an island in the center of the city, is a constant reminder of Paris' history. It has seen more than its share of epic dramas, including the French Revolution and two world wars. But now there is another challenge. Some 854 years after construction began, one of Europe's most visited sites, with about 12 million tourists a year, is in dire need of repairs. Centuries of weather have worn away at the stone. The fumes from decades of gridlock have only worsened the damage. "Pollution is the biggest culprit," says Philippe Villeneuve, architect in chief of historic monuments in France. "We need to replace the ruined stones. We need to replace the joints with traditional materials. This is going to be extensive."

It will be expensive too, and it's not at all clear who is prepared to foot the bill. Under France's strict secular laws, the government owns the cathedral, and the Catholic archdiocese of Paris uses it permanently for free. The priests for years believed the government should pay for repairs, since it owned the building. But under the terms of the government's agreement, the archdiocese is responsible for Notre Dame's upkeep, with the Ministry of Culture giving it about €2 million ($2.28 million) a year for that purpose. Staff say that money covers only basic repairs, far short of what is needed. Without a serious injection of cash, some believe, the building will not be safe for visitors in the future. Now the archdiocese is seeking help to save Notre Dame from yielding to the ravages of time.