Iceland Ice Cave
The highest elevations on Iceland are covered by the Vatnajökull glacier, an ice cap that sits nearly above the heart of the hotspot that helped form the island. A combination of geothermal heat from the volcanoes and summer melting can produce enough water to flood through the base of the glacier and erode away the ice, sometimes creating a massive outburst flood called a jökulhlaup. If the winter is cold enough, these caves will empty and can be entered and toured with local guides. This cave was photographed in 2015; the pale blue light is likely a tiny hint of blue sunlight able to travel through the ice layer above.
-JBB
Image credit: Diana Robinson (Creative Commons licensed)
https://flic.kr/p/r5AUbS