A scientist from the US space agency believes it is more than a mile long - and only 10% visible.
NASA spots mysterious near-perfect rectangle iceberg in Antarctic
The oddly satisfying phenomenon was discovered near the Larden C ice shelf in the Antarctic Peninsula.
From yesterday's #IceBridge flight: A tabular iceberg can be seen on the right, floating among sea ice just off of the Larsen C ice shelf. The iceberg's sharp angles and flat surface indicate that it probably recently calved from the ice shelf. pic.twitter.com/XhgTrf642Z— NASA ICE (@NASA_ICE) October 17, 2018
A photo shows a thick block of ice up to a mile long dramatically protruding from a sea of thin frozen water, thought to have recently splintered off.
Scientists took the snap from a plane used to monitor changing land and sea ice in the South Pole.
From yesterday's #IceBridge flight: Heavily crevassed land ice in the northern Antarctic Peninsula. pic.twitter.com/f4aGvjrIgh— NASA ICE (@NASA_ICE) October 17, 2018
Many have remarked about its peculiar shape, with some suggesting the work of Photoshop or aliens.
But NASA ice scientist Kelly Brunt explained the process that caused it is fairly common.
There are two types of iceberg, she said.
The most familiar are those that look like prisms, such as the one that sank the Titanic.
The second are called "tabular icebergs", Ms Brunt said, comparing their formation to a fingernail growing too long and breaking off at the end.
"What makes this one a bit unusual is that it looks almost like a square," she told the LiveScience website.
The size was hard to guess, Ms Brunt said, but suggested it was likely more than a mile long.
And as with all icebergs only 10% of it is visible; the rest is buried below the surface of the water.
That was not the only shape that scientists spotted last Wednesday.