Glacial lakes include proglacial lakes, subglacial lakes, finger lakes, and epishelf lakes. During the Llanquihue glaciation Villarrica produced pyroclastic flow deposits, subglacial andesite lavas and dacite dykes. Pillow basalt is also produced by some subglacial volcanic eruptions. Antarctica has several subglacial lakes underneath its ice sheet. However, West Antarctic Volcanoes form underneath ice sheets, and are thus categorized as subglacial volcanoes. Wherever a subglacial tunnel began infilling, long winding formations known as eskers would form. The causes the formation of subglacial lakes, the largest being Lake Vostok in Antarctica. The morphology of subglacial lakes has the potential to change their hydrology and circulation patterns. The role of subglacial lakes on ice dynamics is unclear. Tunnel valleys were formed by subglacial erosion by water. No direct sampling of subglacial lakes has been attempted on the Greenland Ice Sheet. The hole did not reach bedrock, but terminated at a subglacial river. Scientists are particularly worried about accidentally contaminating the subglacial lakes with life forms from outside. They served as subglacial drainage pathways carrying large volumes of melt water. Subglacial volcanism is often characterized by ice melt and subglacial water.