Biržai Castle and Sinkhole Lake

Biržai Castle is an elegant Renaissance-Baroque fortress complex sitting beside Lake Širvėna — a body of water that occupies a massive sinkhole formed by the collapse of underground karst cavities. The castle, originally built in the 16th century by the powerful Radziwiłł family, has been reconstructed and houses a regional museum. But the real fascination of Biržai lies underground. The region sits atop extensive gypsum and dolomite karst formations, making it Lithuania's most geologically active area. Sinkholes appear regularly — some suddenly and dramatically. The surrounding countryside is dotted with sinkhole lakes, disappearing streams, and caves. The Kirkilai sinkholes nearby are a cluster of vivid turquoise pools formed by recent collapses.