WASHINGTON - NASA scientists are watching a giant crack forming over a vulnerable Antarctic glacier and they think it will soon break off into an iceberg the size of New York City.

A close-up view of the crack spreading across the ice shelf of Pine Island Glacier shows the details of the boulder-like blocks of ice that fell into the rift when it split, from this handout image captured by the Digital Mapping System (DMS) aboard NASA's DC-8 October 26, 2011. [Photo/Agencies]

Experts said they noticed the crack on October 14 while mapping Antarctica's Pine Island Glacier with a low-flying plane. The crack is about 18 miles long and averages about 260 feet wide. Scientists say it is growing about 6 feet wider each day.

The Pine Island Glacier is called "the weak underbelly of Antarctica," and it's thinning. But scientists say this type of cracking happens naturally every decade and is not related to global warming. They said the new iceberg could break away by the end of this year or early next.