The region around Biržai is famous for its karst sink-holes, formed when underground water sources wash away the gypsum supporting the soil. More than 9000 of these holes, including the 12.6m-deep 'Cow's Cave' (so-called because an unhappy bovine was swallowed up when it opened, 200 years ago) can be found in the area, many within the 146 sq km regional park. The park office has more information on the topography, plus walks, tours and other activities.
