Over a matter of days in late September, Stef Lhermitte watched via satellite as a new, massive crack formed along the edge of Antarctica’s Pine Island Glacier.

Just last year that glacier shed a Manhattan-sized slab of ice. But that particular iceberg was relatively small.

Lhermitte, a geoscientist specializing in remote sensing at the Netherlands’ Delft University of Technology, expects this latest rift, when it eventually breaks, to produce an iceberg roughly 30 kilometers wide by 10 kilometers across (19 miles by 6 miles).

That would be Pine Island’s sixth-largest calving event since 2001 — producing an iceberg five times the size of Manhattan.

“It is impossible to forecast, but I would expect it to calve somewhere this Antarctic summer [U.S. winter], but it is difficult to further fine-tune it,” Lhermitte said over email.

This calving event wouldn’t be record-breaking, nor an immediate red alert. But it unquestionably perpetuates a troubling trend.

Like most of West Antarctica’s ice shelves — the ends of glaciers floating over the ocean — Pine Island is retreating inland and thinning at an accelerated pace, said Lhermitte.

These ice shelves are hugely important; they hold back immense masses of Antarctic ice from flowing into the ocean, just like a plug or cork.

And this is a cork you don’t want to remove.

“They’re like the cork in a bottle,” Josh Willis, a NASA oceanographer who researches glaciers from aboard aircraft, said in an interview. “If you break off a shelf, they [glaciers] can speed up very quickly.”

Pine Island is already breaking off more ice than it can replenish.

This means the ice plug is retreating back to land, ultimately becoming more vulnerable to weakening, or collapse. This would likely let loose rivers of ice into the sea, which would eventually mean yards — not feet — of sea level rise.