The team had been working on the baroque dome of the cathedral in Valencia for more than a month, removing gray paint and fending off birds flying in and out of the hole, Valencia's regional government said Thursday.
Underneath, the experts had been hoping to find Renaissance artwork cited in centuries-old cathedral records, although they feared it might be ruined. Their stroke of serendipity came Tuesday as they were drawn to the hole by the pigeons and their cooing.
One of the team leaders, Javier Catala, stuck a digital camera inside, shot blindly and came back with partial but spectacular images of a well-preserved fresco believed to be more than 8 meters (26 feet) in diameter.
The photos show parts of four winged angels against a starry blue background, all surrounded by gold-leaf trim.
The baroque ceiling turned out to be a false one that masked a fresco completed by Italian painters Francesco Pagano and Paolo de San Leocadio in 1481. They were hired by papal envoy Rodrigo Borja, a Spaniard who went on to become Pope Alexander VI.
The space between the ceiling and the fresco was 80 centimeters (32 inches) at its widest point, providing plenty of room for a makeshift bird's nest.
The duo of Italian artists served as official Vatican painters throughout Alexander VI's papacy. Before becoming pope, Alexander was archbishop of Valencia and employed them to do paintings in churches in southeast Spanish.
But the fresco is important because it's one of the earliest examples of Italian Renaissance art being imported to Spain, said Fernando Lopez, an art historian who works at the Valencia government's main library.
It is also remarkable because the fresco technique — watercolors painted on wet plaster — was rare in Spain at the time and the one in Valencia is in such good shape, Lopez said.
Normally, baroque artists covering up an existing work would scrape it off. "This time they did not. They left an air pocket," Lopez said. "That is the big surprise."
Covering up one kind of art with another simply reflected shifting tastes over the centuries, not a deliberate snub, said Carmen Perez, another art historian who was in on the find.
"The ones who have a reputation for following fashion are women. But art follows fashion more than we do," she said.
Italian art expert, writer and researcher Stefano Sieni said Pagano and de Leocadio were minor figures but typical of roving Renaissance artists — "sponges that took in what was around them, the techniques, the artistic and social influences" — and carried them abroad.
"They had their market, their supporters, but were not great masters," Sieni said in Rome.
The baroque work covering the fresco was ordered in 1674 when church officials deemed that smoke from candles had darkened the fresco, the Valencia government said.
Records show that when the Italians finished the fresco, church officials didn't like it and refused to pay the agreed fee of 3,000 gold ducats. The painters appealed to the governor of Valencia and won.
The cathedral in Valencia dates back to 1262, but it wasn't completed until the late 18th century.