Today, the city of Pisa in Italy is undoubtedly best known for its Leaning Tower, but during the Middle Ages, Pisa was a thriving seaport, home to Portus Pisanus, one of the most influential harbors of its time.

To anyone who has visited this historic town, that might come as a surprise, because Pisa isn't directly on the coast; the ocean is about a 20-minute drive away. For this reason, Portus Pisanus is commonly referred to as "the lost harbor of Pisa." At some point hundreds of years ago, the coastline must have shifted dramatically, transforming this iconic city's landscape.

The mystery of what happened to Portus Pisanus has long perplexed historians. But now, a new analysis of historical maps combined with geological data has finally begun to piece together the morphology of the coast around the Pisa harbor basin, reports Phys.org.

The research team, led by David Kaniewski, also took into consideration extensive biological samples from sediment layers, which helped to reveal how seawater, freshwater or agricultural activities may have influenced the environment in the area. What they found was evidence that a great lagoon with a seaworthy connection to the ocean had formed just south of Pisa's borders around the year 200 B.C. It was on the edge of this lagoon where evidence of Portus Pisanus can be found.

It must have been a spectacular sight to behold — an enormous inland lagoon bustling with traders from around the Mediterranean — far different from the landscape now. But researchers found that by 1000-1250 A.D., the lagoon's connection to the ocean began to diminish, and by 1500 A.D. it would have closed completely, and the lagoon basin became a coastal lake instead.

"Our results underline the importance of such approaches to understand the role of long-term coastal changes and their impacts on the societies living by the sea, notably in the last two millennia," said Dr. Matteo Vacchi from the University of Exeter.

Given that about 40 percent of the world's population lives within 100 kilometers of a coastline, what happened to the lost harbor of Pisa is a chilling reminder of what could happen to coastal cities and the great harbors around the world today. Our coastlines are extremely dynamic, and highly sensitive to climate change.

"The study of the evolution in the coastal zone in the past is a fundamental tool to predict future changes in the context of climatic change," echoed Vacchi.

Scientists now know what happened to the once-great lost harbor of Pisa, Italy
The Leaning Tower of Pisa would have overlooked a magnificent seaport.