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.

Now and then, 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.

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