Doyle Rice

USA TODAY

It's the ice crack that's captivating the world.

One of the world's biggest icebergs ever is poised to break off from an Antarctic ice shelf, but scientists say it's still hanging on by a 12-mile "thread."

They also aren't sure when the now 110-mile crack will finally break open the rest of the way, creating a massive iceberg larger than Rhode Island. "It is particularly hard to predict when it will occur," said Adrian Luckman of Project MIDAS, a British Antarctic research project that's keeping watch on the ever-growing crack.

"I am quite surprised as to how long it is holding on!" he said in an e-mail to USA TODAY.

The crack in the Larsen C Ice Shelf is more than 1,000 feet wide and has grown by 50 miles since 2011, according to the British Antarctic Survey. Once the crack goes all the way across, the iceberg will break off.

The largest icebergs known have all broken off from ice shelves, the survey said.

Ice shelves are permanent floating sheets of ice that connect to a land mass, according to the National Snow and Ice Data Center. (Thus, since they're floating, when the berg finally breaks off, it won't add to sea-level rise). However, ice shelves also serve to hold back the ice behind them: When ice shelves collapse, the ice that had been trapped behind it plops into the ocean, where it then adds to sea-level rise.

Most of the world's ice shelves hug the coast of Antarctica. The Larsen C shelf is on the Antarctic Peninsula, the portion of the continent that juts out toward South America. The Larsen ice shelf used to have three parts: A, B and C. Larsen A and Larsen B collapsed in 1995 and 2002, respectively. And now it's Larsen C's turn.

"The rift (or crack) has continued to open, and the berg continues to drift outward at a very consistent rate," Luckman said. However, he added that the crack has not grown longer in the past several weeks.

There is not enough information to know whether the expected calving event on Larsen C is an effect of climate change or not, although there is good scientific evidence that climate change has caused thinning of the ice shelf, according to Project Midas. In the past 50 years, the Antarctic Peninsula has experienced extraordinary warming of more than 4 degrees, the European Space Agency said.

Could the colder temperatures of winter that are on the way delay the calving process? (Calving is the scientific term for the process by which the berg shears off.)

No, Luckman said. "Calving is not dependent at all on air temperatures," he said. "The ice that is fracturing is buried deep in the shelf and does not feel the change in seasons. If anything, I would say that winter might make the calving more likely because of mechanical stresses caused by wind and waves.

"But I should stress again, this is not a predictable process because we know only a little about the nature of the ice," he said. "It could go today, or it may be months."

Once the iceberg shears off, the 2,000-square-mile object should float along the coast of Antarctica, then head out into the Southern Ocean and eventually break apart into smaller chunks that would melt into the ocean.