ONE BIG, BAD ICEBERG

After a quarter-century with the British Antarctic Survey, Mike Thomson is not easily impressed by icebergs. But the one that showed up in satellite photos earlier this year was the biggest he'd seen in years. Floating slowly out to sea off the Antarctic Peninsula, the frozen slab was about 600 ft. thick, 23 miles wide and 48 miles long. It was a megaberg roughly the size of Luxembourg.

Yet it was not until Thomson and his colleagues hopped aboard a plane this month for a closer view of the monster that they were truly shocked. The appearance of the iceberg was...