The Biržai region is renowned for its impressive natural features, and one of the most intriguing phenomena is the karst sinkholes that form here due to underground water activity. When water erodes soluble layers of gypsum, marl, dolomite, chalk, or salt in the depths, the ground collapses, creating what are known as karst sinkholes. There are about 9,000 sinkholes in the Biržai region, and their number continues to grow.
Karajimiškis Landscape Reserve.
Just 3 kilometers west of Biržai, you'll find the Karajimiškis Landscape Reserve, where the density of karst sinkholes is highest – more than 200 per square kilometer. Here is also the largest and most famous sinkhole – Cow Cave.
Cow Cave: A Geological Monument.
Cow Cave is the most well-known and thoroughly studied geological monument in Northern Lithuania.
It was designated as a national natural monument in 1964. A wooden post with carved inscriptions "Cow Cave" and "Natural Monument" stands at the edge of the sinkhole. The sinkhole is almost circular and funnel-shaped, and speleologists estimate it to be about 200 years old. Its diameter reaches 10–12 meters, and its depth is about 12.6 meters. At the bottom of the pit, there is an open cavity that hides a 9-meter-deep karst depression with five branches: Wet Cave, Narrow Passage, Bat Cave, Toad Cave, and a 1.5-meter-deep underground lake.
Legends of Cow Cave.
Cow Cave is constantly changing – its parameters, cave lengths, sinkhole shapes, and other voids are active and dynamic. According to legend, a cow once fell into this sinkhole, leaving only the end of the chain on the surface as a testament to the event.