Grotto Calgeron, Natural cave in Grigno, Valsugana, Italy

Grotto Calgeron, also called Grotta G.B. Trener, is a cave near Grigno in Valsugana, Trentino. Its 3,600 metres of mostly horizontal passages are cut into dolomite, with 11 ponds and terraced pools of white concretion near the entrance. The cave runs from +250 m to -130 m, and its seventh lake, Lago Pasa, is 4 m deep and 40 m long.

The cave formed during the Riss glaciation period roughly 250,000 years ago, as shown by white clay layers preserved inside. This ancient origin reveals how long water and geological forces have been shaping this location.

The name Calgeron, or Calderon, is Trentino dialect for the large cylindrical pots once used locally. It refers to circular erosion hollows on the cave floor, visible in the left side branch, where stones spun in strong currents and ground round holes; the smoothed stones that formed them still lie inside. The cave is also called Grotta G.B. Trener.

From the car park at Selva di Grigno a path climbs and then follows the mountainside to the entrance at 470 m, in just over 30 minutes. The gate is usually left open, but a guide is advisable. Just past it lie one or two pools, best crossed by small dinghy or in a wetsuit. Water can close the entrance and the siphon beyond the sala delle vasche.

The seventh lake inside the cave, Lake Pasa, is 40 metres long and 4 metres deep. The cave also holds a chamber 80 metres long, and its passages are cut into dolomite rather than limestone. A white clay layer suggests the cavity dates to the Riss glaciation, around 250,000 years ago.