In Furnas, on the island of São Miguel, water does not emerge from the earth in a single form. Some springs surface cold and filled with carbon dioxide. Others rise hot, iron-rich, acidic, sulfuric, or nearly boiling. Together, they form one of the most extraordinary hydrothermal landscapes in Europe — a place scientists describe as one of the continent’s great “hydropoles.”

The term is not poetic invention, although it sounds as if it belongs to one. It is a geological and hydrochemical classification rooted in the remarkable concentration and diversity of mineral and thermal springs found within the caldera of the Furnas Volcano. According to the Azores Natural Parks, the protected landscape covers approximately 3,150 hectares and is considered “one of the largest hydropoles in Europe due to the great variety of mineral and thermal water springs.”

Beneath the steam-filled valley lies a vast underground system shaped by volcanic heat, hydrothermal circulation, mineral-rich rock, and gases rising from deep within the earth. Scientists affiliated with the Institute of Volcanology and Risk Assessment (IVAR), the University of the Azores, CIVISA, the University of Aveiro, and the University of Lisbon described the Furnas volcanic system in a 2025 study published in the journal Water as a complex network of nested calderas, fumarolic fields, craters, domes, thermal reservoirs, and mineral springs.

At the surface, Furnas appears almost mythical: clouds of vapor rising from the ground, the scent of sulfur drifting through the air, hot pools bubbling beside cold springs, mineral-rich waters staining stone with iron and heat. Yet the visible landscape is only the surface expression of a much deeper geological architecture.

According to Portugal’s National Laboratory of Energy and Geology (LNEG), the distribution of Furnas’s springs is closely linked to a deep hydrothermal aquifer located roughly 165 meters underground, where geothermal pressure and volcanic gases — especially carbon dioxide — move upward through structural fractures in the volcano.

That interaction explains why the waters of Furnas are so varied. Rainwater infiltrates volcanic formations high in the mountains, travels slowly underground for decades, heats through geothermal activity, absorbs minerals and volcanic gases, mixes with deeper hydrothermal fluids, and eventually returns to the surface carrying distinct chemical signatures.

The 2025 Water study analyzed 39 mineral springs across the Furnas volcanic system and classified them into three thermal categories: hyperthermal waters between 89.4°C and 95.4°C, thermal waters between 29.9°C and 70°C, and cold waters between 14.2°C and 21.4°C. Most were identified as sodium bicarbonate waters with neutral to slightly acidic pH, though some acidic sulfate-rich waters were also documented.

Researchers concluded that Furnas’s waters are shaped primarily by two geological processes: the leaching of volcanic rock through prolonged water-rock interaction and the influence of volcanic gases and fluids rising from depth.

One of the most significant thermal sources is the Quenturas Spring, also known as Água Férrea das Quenturas. LNEG describes it as an acidic thermal water rich in dissolved gases, with a temperature near 59°C and high mineralization. Isotopic studies suggest the water may spend between 23 and 30 years underground before resurfacing.

Scientists say the Furnas hydrothermal system is more than a tourist attraction or spa destination. It also functions as a natural laboratory for monitoring volcanic activity and groundwater evolution in active volcanic environments. The chemistry of these waters provides critical data about subterranean volcanic processes and long-term environmental change.

The region’s relationship with thermal waters dates back centuries. Studies referenced in academic research from the University of Lisbon note that Furnas has been observed and studied since the 16th century because of its thermal phenomena and therapeutic uses. In 1964, authorities formally established the Furnas Thermal Spa Mineral-Medicinal Concession, encompassing dozens of mineral springs and fumaroles.

Today, the numbers reveal the extraordinary concentration of mineral water resources in the region. Research from the University of the Azores found that the Furnas-Povoação groundwater system contains approximately 283 springs, with 41 mineral-water sites located in the parish of Furnas alone — more than half of all mineral-water points identified on São Miguel Island.

For scientists, geologists, and environmental researchers, Furnas represents a rare convergence of natural heritage, scientific value, and cultural identity. For residents and visitors, however, it remains something even more elemental: a valley where the earth still speaks through water.

The springs of Furnas are not identical because the journeys beneath the volcano are not identical. Each source rises carrying a different story of heat, minerals, gases, pressure, and time. Together, they form a living geological archive — a landscape where the surface beauty of the Azores is inseparable from the invisible volcanic world beneath it.

Translated and adapted from Diário dos Açores – Paulo Viveiros, director.