It is the largest and most famous natural water basin of the Euganean Hills, and it is fed by thermal spring waters, which emerge in pools that are also clearly visible on the surface, at a temperature of about 45°C, allowing the lake to never drop below 17-18°C even in winter. The abundant presence of peat, material derived from decomposing plant remains predominantly from lake or marsh environments, in the surrounding soils, besides giving a characteristic black color to the earth, testifies to the fact that the lake was much larger and has gradually shrunk over time.
These deposits date back to the Quaternary and have been the subject of studies for the presence of fossil pollens from ancient plants that have documented the evolution of climate and vegetation since the Ice Ages. On average, it is about ten meters deep, but at some points, where thermal springs flow in, it can reach depths of 17-19 meters. The lake is important for the high quality of the mud contained within it, which is used for therapeutic purposes, making it the main site for their extraction and use in the thermal complexes of the Euganean area. From a historical perspective, it is significant for the interesting discoveries of archaeological artifacts that testify to the presence of pile-dwelling settlements dating back to the Bronze Age. It was also known as the "lake of the seven fountains" for the presence of numerous cold, hot, salty, and sulfurous springs that fed it. Probably, an episode from the "Last Letters of Jacopo Ortis" is set right in this place as it describes a walk of Jacopo and Teresa "along the shore of a little stream to the lake of the five springs."
Around the site, many legends and folk tales were born, particularly linked to the mysterious aspect it takes on in the winter season when the smoke produced by the steam of its warm waters covers it with a suggestive halo.
