Grotte di Frasassi: Italy's Most Spectacular Underground World

On 25 September 1971, a team of speleologists from the Gruppo Speleologico Marchigiano lowered themselves through a hole in the limestone rock of the Marche Apennines. They had expected a small cave. What they found instead was the Abisso Ancona — a single chamber 240 metres long, 120 metres wide, and 200 metres high, completely filled with forest of white and cream-coloured stalactites and stalagmites. It is large enough to contain the entire Milan Cathedral. They were the first humans ever to see it. The Grotte di Frasassi had been forming in complete darkness for over 190 million years.

The caves extend for at least 30 km through the limestone mountain — only about 1.5 km is open to the public, but that 1.5 km is extraordinary. The main chamber, the Abisso Ancona, gives way to a sequence of smaller caverns, each with its own character: the Hall of Candles (where thin stalactites no thicker than a pencil hang in their thousands), the Room of Infinity (named for its perceived endlessness), and the Great Cascade, a solidified 'waterfall' of calcite 14 metres high. The formations build at a rate of about 1 cubic centimetre per century. The largest columns took millions of years to form.

What makes Frasassi unusual among cave systems is not just scale but variety. Some sections feel ethereal — white and luminous, like the interior of a cloud. Others are amber and rust-coloured, stained by iron and manganese. The cave contains its own ecosystem: a unique species of shrimp (Niphargus frasassiensis) discovered here and found nowhere else on Earth, living in the underground stream that still flows through the lower levels. The cave air is also unusual: permanently 14°C and 99% humidity, which means you will be damp within minutes. Bring a jacket.

The geology of Frasassi is particularly interesting. The caves were not formed primarily by slightly acidic rainwater dissolving the limestone from above — the conventional mechanism for most cave systems. Instead, they were formed by sulfuric acid rising from below, where hydrogen sulfide from deep underground reacted with oxygenated water near the surface. This rare bottom-up process, called 'hypogenic speleogenesis', produces exceptionally large chambers and a distinctive suite of cave minerals not found in conventionally formed caves. It is the same process that created the famous Lechuguilla Cave in New Mexico — which means Frasassi and Lechuguilla are geological cousins, formed by the same unusual chemistry on opposite sides of the Atlantic.