Quick, think of an iceberg. What probably popped in your mind is a mountain of frozen white bobbing menacingly in on a polar sea—like the one that sank the Titanic.
But that’s just the tip of the iceberg, literally. Physics tells us that up to 90% of these giant ice-slabs sit beneath the surface. And every once in a while, we see another reminder of that phenomenon when an iceberg flips upside-down. Like this one, captured in December by filmmaker Alex Cornell:
The vitreous blue-green gleam is the handiwork of decades of pressure that squeezed out the tiny air pockets that buffer the crystals. Ice that dense absorbs bits of red light, reflecting the blue frequencies. Underwater microorganisms and minerals frozen within burnish its greenish tint. The part of the iceberg we normally think about, the part that typically pokes above the sea, is frosted over with layer upon layer of relatively less-densely packed snow. Here’s a comparison:
Anyone who’s left a bottle in the freezer too long knows all too well that ice expands. That expanded mass makes icebergs less dense than the sea around them, letting the upper tenth float above the surface.
But sometimes in stormy weather or as an iceberg cleaves from the glacier—a process called “calving”—it flips. Or, as an iceberg drifts into warmer seas, the melting of its massive underbelly can throw it off balance, upending it.
Though this somersaulting iceberg phenomenon is rare, it is happening more frequently as the sea and atmosphere warm, as Justin Burton, professor at Emory University, told Smithsonian.com. One common source of icebergs are “outlet glaciers,” tongues of ice that are still attached to the glacier sitting on Antarctica’s land mass. When a chunk of ice gently breaks free from the end, an iceberg is formed. As these outlet glaciers retreat, icebergs are now snapping off where the ocean hits the land—and they’re so thin that they flip, Burton explained.
Capsized icebergs are still uncommon enough that when Cornell first encountered the glassy blue berg—as his ship explored Cierva Cove, along the tail-like end of Antarctica that flicks up toward Cape Horn—he couldn’t tell what he was looking at. The naturalist on board explained what it was.
Though Cornell says he’s reluctant to guess just how big the upside-down iceberg was, it was a lot taller than he was. In general, small bergs—ones about the size of a VW Bug—are called “growlers,” while house-sized ones are called “bergy bits.”
And they can get even larger than that. The biggest iceberg ever recorded towered 550 ft. (168 m) above sea level. Imagine what its underside looked like.