The speleologists held their breath when the stone began to fall in the total darkness.
The echo came after a period of time that seemed like an eternity: it was 27 September 1975, and the Frasassi Caves, one of the most spectacular Karst complexes in the world, had just been discovered.
Today the caves are open to the public and the public can visit them, accompanied by professional guides, along an easy tourist route that lasts about 75 minutes.
Visitors can experience the thrill of a hidden and beautiful upside down world, made of breath-taking scenery, rich in extraordinary concretions, where the silence is broken only by the dripping of the drops of water that bring the constantly changing complex to life.
Subject to booking, there are two adventure trails - the blue trail (for those over the age of 12) and the red trail - which allow small guided groups to pass through underground passages, narrow pathways and shafts and admire splendid rooms concealed from the gaze of "normal" tourists.