Complex sentence: An underground spring in Nazareth traditionally served as the city's main water source for several centuries, possibly millennia; however, it was not always referred to as "Mary's well" or "Mary's spring".
Split into simpler sentences: An underground spring in Nazareth traditionally served as the city's main water source for several centuries, possibly millennia. However, it was not always referred to as "Mary's well" or "Mary's Spring".