A state-protected geological monument in Lithuania, Žalsvasis Spring attracts the interest of both scientists and nature lovers due to its uniqueness and geological significance. Listed in the Lithuanian Record Book, this sinkhole is known as the deepest cave in the country. This natural wonder, with a diameter of about 5 meters and a depth of 8 meters, later branches into two underground halls – the "Sunny" and the "Aenigma." The cave’s longest measured section is 21 meters, making it a significant subject for geological research.
The sinkhole's history dates back to around 1960 when the spring emerged. Local residents previously heard underground rumblings, and the collapse of the ground
deformed nearby infrastructure. The spring water is clear, has a sulfurous odor, and is very cold – its temperature remains constant at 7-8 degrees Celsius year-round. The water flows abundantly, with about 10 liters per second discharging from the spring into the river. During winter, this place becomes a haven for birds, as the constant water temperature keeps the river from freezing.
Žalsvasis Spring is not only a natural wonder but also a historical site, reminiscent of Pasvalys' past glory as a resort town known for its springs and sulfur baths. This heritage revives the history of Pasvalys, reminding us of the times when the town was renowned as a center for health and recreation.