This is an ice cave, known as “crystal Cave” on the frozen lagoon of the Svínafellsjökull glacier in Skaftafell, Iceland. This snow, which is up to 1000 years old, has metamorphosed into highly pressurized glacier ice that contains almost no air bubbles. The lack of air means that it absorbs almost all visible light, apart from the blue fraction which is then visible to the naked eye. This cave in the glacier ice is the result of glacial mill, or Moulin, where rain and melt water on the glacier surface are channeled into streams that enter the glacier at crevices. The water melts a hole into the glacier while the ponded water drains towards lower elevations by forming long ice caves with an outlet at the terminus of the glacier. This cave can only safely be accessed in winter, when the lagoon is thoroughly frozen over.