In Pustlaukis Geomorphological Reserve, rare relief forms are preserved – thermokarst pits, which have appeared as the blocks of ice buried underground melted. People call them “devil’s pits”. Here, there are more than 100 of them. The most impressive of these sinkholes is the Pustlaukis pit – a deep drop-shaped scoop, 100 meters in diameter, with the slopes of 18 meters of height. On the southern and eastern slopes of the scoop, there is a prominent terrace, and at the north end there is a water channel. Pustlaukis pit was formed by a steep waterfall of a swirling stream from the edge of the glacier in the late ice age. The remains of ice in the pit have made the relief shape even more prominent. Once the remains melted, a deep lake emerged, which later became a swamp.