A subterranean void measuring over two million cubic meters, easily large enough to house the entire Milan Cathedral: this is the Ancona Abyss, the colossal opening chamber of the Frasassi Caves. The karst system was revealed to the world on September 25, 1971, when Rolando Silvestri, part of an expedition led by Giancarlo Cappanera, found a hole the size of a steering wheel on the slopes of Mount Valmontagnana in Genga. By dropping a stone into the absolute darkness, the explorers initially calculated a sheer drop of over 125 meters. Formed roughly a million years ago when the cold waters of the Sentino river mixed with rising sulfurous springs, the resulting Grotta del Fiume-Grotta Grande del Vento complex now stretches for over 30 kilometers beneath the Marche region.
What to see
The cave system is a showcase of geological artistry, built over 190 million years of limestone stratification as calcium bicarbonate crystallized into rock. The journey begins in the Ancona Abyss, which spans 180 by 120 meters and rises 200 meters high. It leads into Sala 200, a corridor centered around the Obelisco, a towering 15-meter-tall stalagmite. The Sala delle Candeline features small cylindrical stalagmites encircled by rock rings, resembling candles on saucers, while the Sala Bianca is defined by gleaming layers of pure calcite. Visitors also pass through the Sala dell'Infinito, an irregularly circular chamber supported by massive speleothem columns where the first explorers famously lost their bearings. The individual formations bear highly specific names: the Spada di Damocle (Sword of Damocles) is the largest stalactite at 7.4 meters long and 150 centimeters in diameter. Others include the pale pink Fetta di pancetta (Slice of Bacon), the pure white Fetta di lardo (Slice of Lard), and the Canne d'organo (Organ Pipes), conical-lamellar formations that resonate when struck. The Grotta delle Nottole hosts a sizable bat colony, while deep 25-meter cylindrical wells and subterranean pools like Lago Verde dot the route. Above ground, near the entrance, sit two cliffside sanctuaries: the 1029 Santuario di Santa Maria infra Saxa and the 1828 Neoclassical Tempietto del Valadier.
Visiting
The caves maintain a strict microclimate year-round, with a constant temperature of 14 degrees Celsius (57 degrees Fahrenheit) and relative humidity near 100 percent. Because natural sunlight never penetrates the caverns, they are illuminated entirely by cold white artificial lights to protect the formations. The site opened to the public on September 1, 1974, accessed via a tunnel meticulously engineered by Aldo Neroni Mercati. Beyond tourism, Frasassi has hosted extreme scientific research. In 1986, Italian sociologist Maurizio Montalbini spent 210 days isolated underground for a chronobiology experiment, surviving on pills, honey, and cigarettes; he emerged believing only 79 days had passed. More recently, Penn State microbiologists Jennifer Macalady and Dani Buchheister sampled alien cave goo from Lago Verde, studying how microbial biofilms survive without sunlight or oxygen to understand the origins of life on Earth.
Our editors' tip
Before you enter the Grotta Grande, take a moment to look at the display board near the entrance alongside the memorial plaque for engineer Aldo Neroni Mercati. It shows the original 1970s development plans, which included an open-air return tunnel that was ultimately never built, offering a fascinating glimpse into the logistical challenges of opening this subterranean world.