Beneath the turquoise waters of Lake Ohrid, the "Pearl of the Balkans", scientists have uncovered what may be one of Europe's earliest sedentary communities, and are trying to solve the mystery of why it sheltered behind a fortress of defensive spikes.
A stretch of the Albanian shore of the lake once hosted a settlement of stilt houses some 8,000 years ago, archaeologists believe, making it the oldest lakeside village in Europe discovered to date.
Radiocarbon dating from the site puts it at between 6000 and 5800 BCE.
"It is several hundred years older than previously known lake-dwelling sites in the Mediterranean and Alpine regions," said Albert Hafner, a professor of archaeology from Switzerland's University of Bern.
"To our knowledge, it is the oldest in Europe," he told AFP.
The most ancient other such villages were discovered in the Italian Alps and date to around 5000 BCE, said the expert in European Neolithic lake dwellings.
Hafner and his team of Swiss and Albanian archaeologists have spent the past four years carrying out excavations at Lin on the Albanian side of Lake Ohrid, which straddles the mountainous border of North Macedonia and Albania.
The settlement is believed to have been home to between 200 and 500, with houses built on stilts above the lake's surface or in areas regularly flooded by rising waters.
Fortress of spikes
And it is slowly revealing some astonishing secrets.
During a recent dive, archaeologists uncovered evidence suggesting the settlement was fortified with thousands of spiked planks used as defensive barricades.
"To protect themselves in this way, they had to cut down a forest," said Hafner.
But why did the villagers need to build such extensive fortifications to defend themselves? Archaeologists are still searching for an answer to the elusive question.
Researchers estimate that roughly 100,000 spikes were driven into the bottom of the lake off Lin, with Hafner calling the discovery "a real treasure trove for research".
Lake Ohrid is one of the oldest lakes in the world and has been around for more than a million years.
Assisted by professional divers, archaeologists have been picking through the bottom of the lake often uncovering fossilised fragments of wood and prized pieces of oak.
'Like a Swiss watch'
Analysis of the tree rings helps the team reconstruct the daily life of the area's inhabitants – providing "valuable insights into the climatic and environmental conditions" from the period, said Albanian archaeologist Adrian Anastasi.
"Oak is like a Swiss watch, very precise, like a calendar," said Hafner.
"In order to understand the structure of this prehistoric site without damaging it, we are conducting very meticulous research, moving very slowly and very carefully," added Anastasi, who heads the team of Albanian researchers.
The lush vegetation at the site makes the work painstaking slow at times.
"Building their village on stilts was a complex task, very complicated, very difficult, and it's important to understand why these people made this choice," said Anastasi.
For the time being, scientists say it is possible to assume that the village relied on agriculture and domesticated livestock for food.
"We found various seeds, plants and the bones of wild and domesticated animals," said Ilir Gjepali, an Albanian archaeology professor working at the site.
But it will take another two decades for the site to be fully explored and studied and for final conclusions to be drawn.
According to Anastasi, each excavation trip yields valuable information, enabling the team to piece together a picture of life along Lake Ohrid's shores thousands of years ago – from the architecture of the dwellings to the structure of their community.
"These are key prehistoric sites that are of interest not only to the region but to the whole of southwest Europe," said Hafner.