The valley of the ox
The most dramatic feature on Mount Etna is not its summit, but a massive horseshoe-shaped scar on its eastern flank. This is the Valle del Bove, or "Valley of the Ox," a vast depression measuring 5 kilometers wide and 7 kilometers long, with cliff walls rising up to 1,000 meters. This is not the work of a single eruption, but the remnant of a catastrophic failure of the volcano's side.
Geological evidence points to a massive collapse event around 8,000 years ago (or between 7478–7134 BCE). An older, larger volcanic structure, known as the Trifoglietto volcano, gave way. The collapse sent a debris avalanche of rock and volcanic material careening into the Ionian Sea. Computer simulations of this event estimate that 25 cubic kilometers of material slid into the water in a matter of minutes. This sudden displacement of water generated a devastating tsunami that radiated across the entire eastern Mediterranean. Evidence suggests its waves reached the shores of Greece and the Levant, possibly contributing to the abandonment of the Neolithic coastal settlement of Atlit-Yam, now submerged off the coast of Israel.
The evidence for this ancient landslide is the valley itself. High-resolution seismic surveys of the seafloor offshore from Etna have revealed a huge field of debris from the avalanche. This deposit, known as the Milo debris avalanche, confirms the scale of the flank collapse.
A volcano on the move
The forces that caused the ancient collapse are still active today. Etna's entire southeastern flank is unstable and slowly sliding into the Ionian Sea. This movement is not directly driven by magma pressure from within, but primarily by gravity acting on the massive, water-saturated volcanic edifice.
An extensive network of GPS stations, satellite radar, and underwater acoustic transponders monitors this movement in real-time. Data shows the flank is moving at an average rate of 2 to 5 centimeters per year. While this seems slow, the movement can occur in sudden bursts. In May 2017, a section of the submerged flank lurched 4 centimeters in just eight days. This movement happened without any corresponding earthquake, a phenomenon known as a "slow slip event."
The northern boundary of this sliding block is defined by the Pernicana Fault system, one of the most active tectonic structures on the volcano. This left-lateral fault accommodates the seaward slip and is characterized by both shallow earthquakes and continuous, non-seismic creep. Scientists constantly monitor this system, as accelerations in its movement can signal changes in the stability of the entire eastern flank.