Zelenci Springs is a nature reserve near the town of Kranjska Gora, in the far northwestern corner of Slovenia. It is the source of the Sava Dolinka River, a tributary of the Danube. At Zelenci Springs, water from underground Nadiža Creek re-emerges through the porous bottom of a 2 m deep lake, whose waters are noted for their deep, brilliant green. The spring and its surrounding area are named after this colour (Zelenci is a deadjectival plural noun from Slovene zelen 'green'.) The porous chalk of the Zelenci Springs lake bed allows the constant upwelling of groundwater in the form of tiny jets, a phenomenon unique in Slovenia. The lake formed by the springs has a constant year-round temperature of 5 to 6 °C. Source: https://en.wikipedia.org/wiki/Zelenci