This is the only place in the Baltic States where the landscape is always changing due to gypsum in the soil being dissolved by underground water, resulting in sinkholes. The most unique feature of the Biržai region is that you never know where and when a layer of the earth that was formed 300 million years ago will open up. It is estimated that there are about 9,000 sinkholes in this region. Karvės Ola (“Cow’s Cave”) is the largest and most popular sinkhole, with a depth of more than 12 metres, and a diameter of 10–12 metres. There is an open hole in the bottom of the depression. There are five caves (Šlapioji Ola, Siauroji Landa, Šikšnosparnių Landa, Rupūžės Ola) and a small underground lake in the bottom of the sinkhole. It is believed that Cow’s Cave appeared 200 years ago. Nearby there is the 700-metre-long Barsukas Educational Trail. As you walk along this trail, you will see a few sinkholes similar to Cow’s Cave.
If you decide to visit Biržai’s pride and joy – the 30-metre Kirkilai Observation Tower – and climb up to the deck, you will be rewarded with impressive panoramic views of 30 water-filled sinkholes known as the Kirkilai Karst Lakes.