One of the most important arctic glaciers is coming to a stop, leaving ice researchers puzzled.

Once the fastest-flowing ice stream in Greenland, moving at 10 miles a year, satellite data show that Jakobshavn Isbrae’s charge to the sea has slowed almost to a halt. Over the last two decades, as it flowed into Disko Bay, the glacier’s front end also retreated and thinned, falling in height by as much as 60 feet per year. But now, Jakobshavn has slowed to a crawl, while its trunk has gotten longer and thicker.

Researchers are not sure whether the glacier, believed to be the source of the iceberg that sank the Titanic, has paused or is coming to a complete stop. However, in the context of global climate change, scientists are worried that the complete reversal of a long-observed dynamic could be the start of a retreat.