Alex Cornell (used with permission). When an iceberg flips over we get to see the beautiful underside

A rare sighting of an upside-down iceberg has been captured on camera, revealing a striking deep blue underbelly that once resided beneath Antarctica’s frigid waters.

Usually icebergs are white because they are made of compressed snow, which reflects all frequencies of visible light. However, if high pressures squeeze the flakes together, or sea water freezes, the gaps between the snowflakes disappear, taking with them the equi-reflecting surfaces.

Long wavelengths of light from the sun (reds and yellows) are absorbed when passing through the ice, whereas blue light is scattered. Some of the scattered light is reflected to us, producing the blue color we associate with pure water–although microorganisms or chemicals can sometimes add a greenish tint.

Credit: Alex Cornell, with permission.

The same thing happens when the water is frozen, provided all the air has been eliminated. This occurs a lot more often at the bottom of large, old blocks of ice than the top, and is usually hidden from view. Most of the time, the only way to see such blue ice is to go swimming in sub-zero temperatures – unpleasant even in the best wetsuits.

Now and then, however, an iceberg flips over, allowing us to see what has happened to its lower reaches. Most often this occurs when the iceberg has just calved, but occasionally it transpires later, for example in a storm.

When visiting Antarctica at the end of last year, filmmaker Alex Cornell came across the aftermath of one such event. He's now selling large print versions of these images, along with more conventional photographs of iceflows, mountains and wildlife.

Cornell has also provided this comparison of an ordinary iceberg set against an upturned one.