This is the edge of a huge tabular iceberg
that broke away from the Ross Ice Shelf in March 2000. The berg
was around 300km long by 40km wide (186 x 25 miles, around twice the
area of the state of Delaware). It has since broken up into several
smaller but still very substantial chunks called B15A, B15B etc.
When first encountered it is difficult to conceive
that such a thing can simply be floating in the sea, it looks like the
edge of some enormous landmass.

Picture courtesy of Mike Usher - Mike went on
a Ross Sea expedition on board the Kapitan Khlebnikov in 2005.
You can purchase a selection of Mike's pictures
here