Archaeologists in eastern Bulgaria say they have unearthed the oldest prehistoric town ever found in Europe and an ancient salt production site that may explain massive riches discovered in the region. Excavations at the site near the modern-day town of Provadia have so far uncovered the remains of a settlement of two-storey houses, a series of pits used for rituals as well as parts of a gate, bastion structures and three later fortification walls - all carbon dated between the middle and late Chalcolithic age from 4700BC to 4200BC. "We are not talking about a town like the Greek city-states, ancient Rome or medieval settlements, but about what archaeologists agree constituted a town in the fifth millennium BC," said Dr Vasil Nikolov, a researcher with Bulgaria's National Institute of Archaeology with Museum. Nikolov and his team have worked since 2005 to excavate the Provadia-Solnitsata settlement, located near the Black Sea resort of Varna. A small necropolis, or burial ground, was also found this year, but has yet to be studied extensively and could keep archaeologists busy for generations. Another institute archaeologist, Dr Krum Bachvarov, called that find "extremely interesting" due to the peculiar burial positions and objects found in the graves, which differ from similar graves found in Bulgaria. "The huge walls around the settlement, which were built very tall and with stone blocks … are also something unseen in excavations of prehistoric sites in southeast Europe so far," Bachvarov said. Well fortified, a religious centre and most importantly, a major production centre for a specialised commodity that was traded far and wide, the settlement of about 350 people met all the conditions to be considered the oldest known "prehistoric town" in Europe, the team says. "At a time when people did not know the wheel and cart, these people hauled huge rocks and built massive walls," Nikolov said. "Why? What did they hide behind them? Salt." The area is home to huge rock-salt deposits, some of the largest in southeastern Europe, and the only ones exploited as early as the sixth millennium BC, Nikolov said. Salt is still mined there, but it had a much more significance 7,500 years ago. "Salt was an extremely valued commodity in ancient times, as it was both necessary for people's lives and was used as a method of trade and currency starting from the sixth millennium BC up to 600BC," he said. Salt extraction at the site first began in about 5500BC when people started boiling brine from the nearby salty springs in dome kilns found inside the settlement, Nikolov said. "This is the first time in southeast Europe and western Anatolia that archaeologists have come upon traces of salt production at such an early age, the end of the sixth millennium BC, and managed to prove it with both archaeological and scientific data," Bachvarov said. Salt production was moved outside the settlement towards the end of the sixth millennium and productivity gradually increased. After being boiled, the salt was baked into small bricks. Nikolov said production increased steadily from 5500BC, when one load from the kilns in Provadia-Solnitsata yielded about 25kg of dry salt. By 4700-4500BC, that amount had increased to 4,000-5,000kg of salt. "At a time when salt was as precious as gold, you can imagine what this meant," he said. The salt trade gave the local population huge economic power, which could explain the gold riches found in graves at the Varna necropolis, dating back to around 4300BC. The 3,000 jewellery pieces and ritual objects have been internationally recognised as the oldest gold treasure in the world, raising questions as to how a culture of farmers and stock-breeders could acquire such wealth.