Grünloch, Limestone sinkhole near Lunz am See, Austria

The Grünloch is a sinkhole at 1,270 m above sea level on the Dürrenstein plateau of the Ybbstal Alps in Lower Austria. The depression is about 500 m across, with a height difference of roughly 50 m at its lowest saddle, the Lechnergraben, and a floor only about 100 m in diameter.

During World War II the Wehrmacht ran a test station in the Grünloch, where vehicle engines were assessed for use in Siberia. Heavily rusted relics from that period can still be seen in the doline. Between 19 February and 4 March 1932 a temperature of −52.6 °C was recorded here.

The name Grünloch refers to the green hollow of this basin in the Ybbstaler Alps. During World War II the Wehrmacht ran a test station here, where vehicle engines were checked for suitability in Siberian conditions; rusted relics from that period can still be seen. Today the site is known for its tundra-like vegetation, where only plants fully covered by snow in winter survive the cold-air lake.

Visiting the sinkhole requires sturdy footwear and preparation for changing conditions, especially when descending into deeper areas. Travelers should be aware that temperatures at the bottom can be much colder than the surrounding area, even on warm days.

The lowest temperature recorded here was −52.6 °C, measured between 19 February and 4 March 1932; a reading of −47.1 °C followed on 25 December 2003. Austria's official ZAMG record, however, remains −37.4 °C at the Sonnblick observatory on 2 January 1905. Such values need arctic air, clear skies, snow cover and no wind at once.