When Solskinstjørni is full, it covers an area of ca. 200 m². Its depth is a metre at maximum. The tarn and outflow stream can show up in all seasons, also in the middle of winter. Its periods with water - commonly 3-4 days - are nonetheless much shorter than the periods when the tarn and stream are empty. Those who wander along the path into Norddalen in summer will most often not see other signs of this mysterious natural phenomenon than a lush, grassy hollow. The tarn is located where it cannot be seen from the farm, therefore there are no systematic recordings of it periods with and without water.
The mystery is not totally unresolvable, however. Studies show that the water supply comes from one or several groundwater springs within the peak Osasnuten, which rises up just to the east of the tarn. It also seems clear that the periods with water supply to the tarn are only slightly related to the amount of precipitation. Even though it might rain for several weeks in a row, this will not influence the tarn. An increase in the barometer with subsequent good weather has an effect, however, but not always enough to raise the water level in the tarn.
Ever since Solskinstjørni was mentioned for the first time in the magazine Hermod in 1824, many theories have been launched for this phenomenon, but none can be said to be fully adequate. The solution to the mystery is probably that several hydrological, meteorological and geological factors work in combination. It is likely in any case that Osasnuten contains much ground water and has enough hollow space to store the water. Some of this groundwater gets tapped into bottles from springs on the south side of the mountain to be sold as Osa Mineral Water.
