Sofia is built directly over 42 natural thermal springs that have flowed for over 2,000 years.

The city of Sofia sits atop a complex geological intersection where the Sofia Valley meets the Vitosha mountain range. This location features a unique network of deep-seated faults that allow groundwater to descend several kilometers, heat up via the Earth's geothermal gradient, and rise back to the surface. There are 42 distinct mineral springs in the city center and surrounding neighborhoods, discharging approximately 480 liters of water per second.Historically, the Serdi tribe of Thracians settled here in the 8th century BC specifically because of these curative waters. When the Romans conquered the area in 29 BC, they renamed it Ulpia Serdica and built extensive thermal baths, some of which are still visible today near the Sofia Central Mineral Baths building. This landmark structure, completed in 1913, served as the city's public bathhouse for decades and remains an architectural icon of the capital.The water temperatures range from 37°C to 50°C and contain high concentrations of carbonate, sulfate, and sodium. Despite the massive potential for renewable energy, current estimates suggest that Sofia utilizes less than 10% of its geothermal capacity for heating buildings. Modern researchers from the Bulgarian Academy of Sciences continue to study these reservoirs to determine how they can be better integrated into the city's green energy infrastructure without depleting the ancient mineral content.