Buco del Piombo, Show cave and geological formation in Erba, Italy.

Buco del Piombo is a show cave near Erba with a dramatic limestone entrance. Water has carved intricate formations inside the rock over thousands of years.

Human presence dates to the Paleolithic, attested by lithic finds, and Neanderthals used the cave as a flint quarry from at least 60,000 years ago. Cave bear remains show it sheltered Ursus spelaeus until about 15,000 years ago. Masonry in the entrance hall was radiocarbon-dated to the mid-6th century AD, and the site served as a defensive fort, reinforced under Frederick Barbarossa and later restored for the Medeghino.

Inside the entrance hall stood a speleological museum; the source does not say its collections moved to Erba's civic museum. The cave itself yielded complete cave-bear skeletons and Palaeolithic flint tools, and Neanderthals quarried flint here from about 60,000 years ago. Queen Margherita of Savoy visited.

The cave reopened to the public in May 2024 after safety and renovation works, following years of closure since 2011. Access is on private land, so visits are paid and guided; the walkable gallery runs 300 metres into the mountain. The entrance hall is damp and slippery, and the first metres still receive daylight, where ferns grow.

The name comes from the lead-gray color coating the rocks, created by limestone weathering. This distinctive coating gives the cave its visual character.