Legend has it that the grotto hidden among the craggy cliffs on San Marco hill in Sutera in the heart of Sicily holds a treasure chest full of gold coins. In order to find it, three men must dream simultaneously about the precise place to dig.

Treasure or no treasure, the grotto itself is an archaeological gem, its walls adorned with a multi-coloured Byzantine-esque 16th-century fresco depicting Jesus, the Virgin Mary and Saints Paulinus, Luke, Mark and Matthew.

One of the first mountain oratories in the world, it was built by Basilian monks in the 9th century. But time has weathered the frescoes, which have been completely abandoned to the elements. Anyone who is interested can enter through a flimsy gate as there is no key and chain. But the grotto, now overgrown with vegetation, is unknown even to people living in nearby villages.

This neglected corner of Sicily is just one of hundreds of abandoned archaeological sites, monuments and historical buildings across Italy. The country boasts the highest number of Unesco world heritage sites in the world, but according to EU statistics is second-last in Europe for public funding of culture. As a consequence, parts of its immense heritage – which have survived earthquakes and wars – risk being lost to vandalism, negligence and looting.

Facebook Twitter Pinterest Oratorio Bizantino in San Marco, Sutera. Photograph: Lorenzo Tondo

About 100km from the San Marco grotto, also covered in vegetation, are the ruins of an ancient Greek quarry from the 6th century BC, which was mined to build cities and temples, such as those in Selinunte, in the province of Trapani. Columns and capitals are strewn across the ground from when the Carthaginian fleets arrived on Sicily’s coasts in 409BC.

“Today, this site is at the mercy of predators and thieves, who for centuries have looted the area for its precious artefacts,” said Mimmo Macaluso, an EU researcher on Magna Graecia, the name given by the Romans to the coastal areas of southern Italy that were extensively populated by Greek settlers.

“Anywhere else in the world, this site would have been transformed into a museum, attracting millions of visitors. And instead, there it is, a 6th-century BC treasure falling apart before our eyes.”

Sicily is also home to the necropolises of Pantalica, one of the most important protohistorical sites in the world and a Unesco world heritage site. Strewn among the canyons and forests are ancient tombs and dwellings that date back to the 13th century BC and have been defaced with spray paint in recent years.

“There are archaeological sites that have not even been closed off. Anyone can just walk in and bring home an ancient capital,” said Macaluso. “The only way to protect our heritage is to convince institutions to invest more public funds in our work.”

Facebook Twitter Pinterest Sacro Cuore church in Parma. Photograph: Tesori Abbandonati

Even more difficult is to quantify the number of abandoned churches in Italy. According to researchers there are thousands, many expropriated, then abandoned, during Napoleonic rule. Others have been destroyed in earthquakes. Of those that remain, many are in a pitiful condition, home to rat colonies and prey to vandals.

“A few years ago the Italian Episcopal conference conducted a census of churches belonging to the Vatican,” said Luigi Bartolomei, a research fellow from the department of architecture at the University of Bologna.

“There are about 67,000 places of worship run by the church, but there are several thousand others owned by the Italian state and religious organisations in the country. How many of these have been abandoned? It’s difficult to say, because often there is little information available on these organisations and the churches they own.”

A visitor to Naples in the 1800s wrote in his diary that “Naples has more churches than Rome”. In the city’s historic centre alone, there are more than 200 churches, 75 of which are completely abandoned.

Others have become bona fide rubbish dumps. One is the Scorziata church, built in 1579, which boasted striking paintings including the San Giovannino, an eponymous copy of a painting by Caravaggio. The church is closed to the public, and the interior is a heap of debris. Its paintings and sculptures have all been stolen.

Facebook Twitter Pinterest An abandoned church. Photograph: Tesori Abbandonati

According to the most recent stolen artworks bulletin issued by the Carabinieri, in the last year alone, 8,405 items of archaeological interest have gone missing in Italy.

Despite the abandonment and vandalism, many of these churches continue to attract hundreds of visitors and curiosity seekers, fascinated by the decadent atmosphere they evoke. Partially destroyed vaulted ceilings, and altars and pulpits overgrown with vegetation, remind visitors of the exotic temples from Indiana Jones.

Armed with his camera, Federico Limongelli, 35, spends his weekends with his friends exploring abandoned churches, asylums, villas and other buildings. In 2015 they created a Facebook page titled “Tesori Abbandonati” (Abandoned Treasures), which now has more than 50,000 followers.

“It started as a hobby,” Limongelli said. “These are places that have a certain allure, and telling their stories using photography was also a way to draw attention to their abandonment. Many people have begun following us and posting on our page their own explorations of Italy’s abandoned treasures. Often it was not a question of buildings or churches; sometimes it was entire ghost towns.”

The Italian national institute of statistics has counted nearly 6,000 ghost towns in Italy, many of which are medieval villages that succumbed to earthquakes or landslides. Some continue to attract visitors and tourists, despite being covered in vegetation. Others have ended up on eBay.

In 2012, the 25 homes comprising the abandoned medieval village of Patraricca, located at the source of the Arno river, were auctioned off with a €2.5m opening bid.