PISA (REUTERS) - The Leaning Tower of Pisa is surprisingly still standing, surviving numerous earthquakes over the years.
New research says it's the unique interaction between the tower's foundation and the soft soil it stands in that's helped it ride the seismic waves.
University of Bristol professor George Mylonakis said: "As the earthquake waves reach the tower, the tower somehow rides the waves and doesn't move. So the soil moves but the tower doesn't move; like riding the waves. So had the soil been stiff, the waves would have hit the tower and the tower would have possibly if not probably collapsed."
European engineers, including those from Roma Tre University and the University of Bristol, installed seismic instruments around the tower.
They found that the soil below, combined with the tower's height and thick marble walls, provided perfect dynamic soil-structure interaction.
Professor Mylonakis said: "These earthquakes produce waves that do not resonate with the tower, and this is a very significant factor that has contributed to the survival of the structure."
The ground beneath the tower was a pre-historic riverbed, making the soil there softer.
Ironically, it's the same soil that caused the tower to lean precariously at a five-degree angle.
The 14,000-tonne structure was built between 1174 and 1370.
Engineers are now optimistic it will stand tall for generations to come.
"The help that the tower receives from the ground - is so important and so strong that we are optimistic it will survive for hundreds if not thousands of years," said Professor Mylonakis.