Širvėna Lake is the oldest artificial water body in Lithuania. In 1575, it came into existence following the building of a dam across two rivers running through Biržai for protection of both the fortress and the city. Stretched over a swampy area of nearly 400 hectares, the pond reliably protected the castle and the city from the north. Nowadays, the area of Širvėna Lake is around 325 hectares. Following the establishment of a linen factory in buildings of Astravas Manor in 1928, a 525-metre-long and 2.45-metre-wide footbridge has been built over Širvėna Lake to connect the Astravas Manor and the city of Biržai. It is the longest footbridge across a lake in Lithuania. Today, the footbridge is very popular with anglers and newlyweds, local people and visitors. It is a great place to admire views of the city, savour the calm tranquillity of the lake, and take a stroll to Astravas Manor.